NHC complexes of osmium clusters: A structural and reactivity study

Organometallics 2008, 27, 5777–5799

5777

NHC Complexes of Osmium Clusters: A Structural and Reactivity

Study

,

†

‡

§

Craig E. Cooke,* Michael C. Jennings, Michael J. Katz, R. K. Pomeroy, and

,

†

Jason A. C. Clyburne*

Department of Chemistry, Saint Mary’s UniVersity, Halifax NS B3H 3C3, Canada, Department of

Chemistry, UniVersity of Western Ontario, 1151 Richmond Street, London ON N6A 5B7, Canada, and

Department of Chemistry, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby BC V5A 1S6, Canada

ReceiVed June 23, 2008

A carbene transfer agent, 1,3-bis-(2,4,6-trimethylphenyl)imidazol-2-ylidene-silver(I)chloride ([IMes]-

AgCl), was treated with various osmium clusters. Reaction of ([IMes]AgCl) with [Os

Os (µ-H)(µ-Cl)(CO) (IMes)] (1), an osmium N-heterocyclic carbene complex. The reaction of ([IM-

es]AgCl) with the cluster [Os (CO)10(CH CN) ] yielded two products, [Os (µ-Cl)(CO)10(µ-Ag(IMes)]

2) and [(IMes-H)][Os (µ-Cl)(CO)10](µ -Ag)[Os (µ-Cl)(CO)10] (3). Compound (2) is a heterobimetallic

carbene complex resulting from the complete incorporation of ([IMes]AgCl). Transmetalation using

(IMes)AgCl] and the cluster [Os (µ-H) (CO)12] in the presence of trimethylamine-N-oxide resulted in

the formation of [Os (µ-H) (CO)11(n · IMes)] (4), where (n denotes normal carbene bonding, i.e., through

). [Os (µ-H) (CO)10(IMes) ] (5) was formed using the activated cluster [Os (µ-H) (CO)10(CH CN)

and [(IMes)AgCl]. Compound (5) was found to be sensitive to silica gel column chromatography in the

presence of dichloromethane decomposing to [Os (µ-H) (µ-Cl)(CO)11(IMes)] (6), which possessed a

butterﬂy metal skeleton. The ruthenium analogue of (4), [Ru (µ-H) (CO)11(IMes)] (7), was prepared and

identiﬁed spectroscopically but was found to be thermally unstable. Thermolysis of (4) in benzene yielded

four products. The abnormal carbene complex [Os (µ-H) (CO)11(a · IMes)] (10) (a denotes abnormal

carbene bonding through C or C ), an isomer of (4), and {Os (µ-H)(CO)10[(η-C)N(Mes)C NC

(η -C)]} (11), an unusual complex resulting from the activation of benzene and

activation of three C-H bonds on IMes, were isolated in modest yields. Two higher-nuclearity clusters,

Os (µ -C)(CO)14[η-C-η-NC (Mes)]} (8) and [Os (µ-H) (µ -C)(CO)15(IMes)] (9), both of which contain

carbide ligands, were also isolated. {Os (µ-H)(CO)10[(η-C)N(Mes)C

(η -C)]} (12), an analogue of (10), resulted from the thermolysis of (4) in toluene indicates that the

3 2

(µ-H) (CO)10] gave

[

3

9

3

2

3

(

3

4

3

[

4

C

2

4

2

4

3

2

]

4

3

4

3

4

2

H

2

6 2

H

2

(

CH

3

)

2

4 4

(η -C)(η-C)C H

{

5

H

2 2

6

4

5

2

4

2

H

2

NC

6

H

2

(CH

3

)

2

3

(η -C)(η-C)CHC(CH )-

2

2 2

C H

reaction is potentially general and exhibits some regioselectivity. Crystal structure determinations were

completed on all 11 osmium cluster complexes.

Introduction

The investigation of cluster compounds containing hydride

ligands has long been of interest to chemists. The chemistry of

hydrogen, speciﬁcally relating to its desired use as an energy

source, has led to the increased interest in polynuclear metal

complexes, which can take up and release hydrogen under mild

1

Figure 1. Generalized formula of an N-heterocyclic carbene where

R is an organic moiety.

conditions. Materials of this nature are of interest for hydro-

genation catalysts and hydrogen storage, particularly if the

1

complexes can reversibly absorb large amounts of hydrogen.

known as N-heterocyclic carbenes (NHCs), have the general

structure shown in Figure 1. NHCs have also been prepared,

which contain a variety of heteroatoms other than nitrogen (such

Osmium and ruthenium carbonyl clusters are able to undergo

hydrogenation via displacement carbonyl ligands affording

clusters with varied metal and hydride numbers. As expected,

these materials exhibit reactivities that are different from their

corresponding binary carbonyls, in many cases providing stable

complexes.

Carbenes are neutral compounds featuring a divalent carbon

atom with only four electrons in its valence shell. Complexes

prepared from the metalation of imidazolium-2-ylidene, better

2

as C, B, O, etc.) within in the ring. The use of NHCs as ligands

¨

for transition metals was ﬁrst described in 1968 by Ofele and

3,4

Wanzlick. Aside from the pioneering work of Lappert on

transition metal-carbene complexes, few papers on this topic

(

2) Bertrand, G. Carbene Chemistry: From Fleeting Intermediates to

Powerful Reagents; CRC Press: New York, 2002. (a) Pras a¨ ng, C.;

Donnadieu, B.; Bertrand, G. J. Am. Chem. Soc. 2005, 127, 10182–10183.

(b) Krahulic, K. E.; Enright, G. D.; Parvez, M.; Roesler, R. J. J. Am. Chem.

Soc. 2005, 127, 4142–4143. (c) Canac, Y.; Soleilhavoup, M.; Conejero, S.;

Bertrand, G. J. Organomet. Chem. 2004, 689, 3857–3865.

(3) Ofele, K. J. Organomet. Chem. 1968, 12, P42.

*

Corresponding author. E-mail: Jason.Clyburne@SMU.CA.

Saint Mary’s University.

University of Western Ontario.

†

‡

§

Simon Fraser University.

(

57.

1) Adams, R. D.; Captain, B. Angew. Chem., Int. Ed. 2008, 47, 252–

(4) Wanzlick, H. W.; Schoenherr, H. J. Angew. Chem., Int. Ed. Engl.

1968, 7, 141–142.

2

1

0.1021/om800577x CCC: $40.75  2008 American Chemical Society

Publication on Web 10/22/2008

5

778 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

5

had been published prior to the early nineties. However,

following the isolation of the ﬁrst NHC by Arduengo in 1991,

The sterics of phosphines and NHCs are different; NHCs are

fan-shaped and as such can orient themselves via rotation about

the donor atom-metal bond to minimize steric interactions with

other ligands. Phosphines, however, are usually cone-shaped,

and rotation about the phosphorus-metal bound is not typically

hindered. Extensive work with phosphines was completed in

this regard by Tolman, who developed the concept of the cone

2

,6-10

research on metal-carbene complexes expanded greatly.

Free carbenes, as well as those in metal complexes, have been

2

,6,7,11-13

studied extensively.

Complexes of NHCs have been prepared with virtually every

transition metal and many main group elements, a consequence

of the ability of NHCs to bind to both hard and soft metals

1

2,20,23

angle.

To quantify the steric factors characterizing NHC

1

0,12

ligands, Nolan suggests that the NHC shape can best be

described as the volume of a sphere centered on the metal, buried

by overlap with atoms of the various NHC ligands denoted by

making it a very versatile ligand system.

NHCs bond to

metals primarily through σ-donation of the carbene lone pair

to the metal. Carbene bonding was believed to have been purely

σ-donation in nature; however, recent evidence suggests that

2

4,25

%

VBuried.

The volume of this sphere represents the space

1

4-16

around the metal atom that must be shared by the different

ligands upon coordination. The bulkier a speciﬁc ligand, the

larger the portion of the sphere that will be occupied by that

some degree of back-donation may occur.

The extent of

π-back-donation is dependent upon the metal center to which

it is bound; it is believed to be smaller than that of Fischer

1

6,24

1

6

ligand, i.e., the greater %VBuried.

The cone angle and

carbenes. The bond strength of N-heterocyclic carbenes has

been shown to rival phosphines. Imidazolin-2-ylidene, the ﬁrst

type of NHC to be complexed to silver, is an imidazole ring

with substituents at the nitrogen atoms (1,3-position) and with

the singlet carbene located at the 2-position, and has the general

The stability of NHC-metal

complexes is due to the π-donation from the p-orbitals of the

%

VBuried provide a structured approach to steric analysis.

Given the different steric requirements of NHCs, it is expected

that their reaction with clusters will result in a metal complex

shrouded by the carbene when viewed as a space ﬁlling model,

while PR3 ligands are oriented away from the metal centers and

thus are less likely to participate in intramolecular reactions.

8,11-13

structure shown in Figure 1.

1

5,17

A great deal of research has been completed examining the

synthesis of NHC analogues of phosphine catalysts. As such, a

nitrogen atoms to the empty p-orbital of the carbon atom.

NHCs and phosphines share the properities of both being

monodentate two-electron ligands. NHCs provide a versatile

alternative to phosphine ligands, often showing stronger electron-

donating properties while also providing an equally variable

This results in several advantages

including tighter binding (which often limits catalyst decom-

position reactions associated with ligand dissociation) and

large library of palladium and ruthenium complexes containing

8,12,13,18,26,27

NHCs exists;

from these investigations, it was

found that NHCs are often reactive within the coordination

13

sphere, i.e., NHCs are noninnocent ligands. Several reports

8,11-13,18,19

steric environment.

illustrate that NHCs are susceptible to C-H activation on the

28-32

N-substituents.

identiﬁed.

Also, reductive eliminations have been

33,34

One of the most studied systems in this respect

involves mononuclear iridium-NHC complexes.

8,11-13,18

greater thermal stability than that observed for phosphines.

28,29,31,35,36

This

In tertiary phosphines, PR3, a change of the substituent causes

a change in both the steric and electronic environment of the

ligand since the R group is bound directly to the phosphorus

donor atom. NHCs, however, exhibit little change on variation

of the nitrogen-bonded substituent as it is not directly attached

to the donor atom. A change of N-bonded substituents primarily

changes the steric nature of the NHC. It has been proposed that

in order to affect an electronic change in an NHC ligand, it is

reactivity is quite different from that of tertiary phosphines,

which are typically thought of as spectator ligands as they rarely

20

undergo any modiﬁcation during a reaction.

Several research groups have investigated the use of

NHC-silver(I) halide complexes as NHC transfer agents and

have successfully prepared NHC complexes of several late

1

1,12

transition metals by this method.

We choose to use the

8

,10,12,20-22

best to modify the azole ring.

(

21) Fuerstner, A.; Alcarazo, M.; Krause, H.; Lehmann, C. W. J. Am.

Chem. Soc. 2007, 129, 12676–12677.

(22) Chianese, A. R.; Li, X.; Janzen, M. C.; Faller, J. W.; Crabtree,

R. H. Organometallics 2003, 22, 1663–1667.

(23) Tolman, C. A. Chem. ReV. 1977, 77, 313–348.

(24) Cavallo, L.; Correa, A.; Costabile, C.; Jacobsen, H. J. Organomet.

Chem. 2005, 690, 5407–5413.

(25) Viciu, M. S.; Navarro, O.; Germaneau, R. F.; Kelly, R. A. III;

Sommer, W.; Marion, N.; Stevens, E. D.; Cavallo, L.; Nolan, S. P.

Organometallics 2004, 23, 1629–1635.

(

5) Cardin, D. J.; Cetinkaya, B.; Lappert, M. F. Chem. ReV. 1972, 72,

5

45–574.

(

6) Arduengo, A. J., III; Harlow, R. L.; Kline, M. J. Am. Chem. Soc.

1

991, 113, 361–363.

(

7) Arduengo, A. J., III Acc. Chem. Res. 1999, 32, 913–921.

8) Bourissou, D.; Guerret, O.; Gabbai, F. P.; Bertrand, G. Chem. ReV.

2

000, 100, 39–91.

(

9) Lee, K. M.; Wang, H. M. J.; Lin, I. J. B. J. Chem. Soc., Dalton

Trans. 2002, 2852–2856.

10) Herrmann, W. A.; Schuetz, J.; Frey, G. D.; Herdtweck, E.

Organometallics 2006, 25, 2437–2448.

11) Lin, I. J. B.; Vasam, C. S. Comments Inorg. Chem. 2004, 25, 75–

29.

(26) Marion, N.; Diez-Gonzalez, S.; Nolan, S. P. Angew. Chem., Int.

Ed. 2007, 46, 2988–3000.

(

(27) Scott, N. M.; Nolan, S. P. Eur. J. Inorg. Chem. 2005, 1815–1828.

(28) Scott, N. M.; Pons, V.; Stevens, E. D.; Heinekey, D. M.; Nolan,

S. P. Angew. Chem., Int. Ed. 2005, 44, 2512–2515.

(29) Scott, N. M.; Dorta, R.; Stevens, E. D.; Correa, A.; Cavallo, L.;

Nolan, S. P. J. Am. Chem. Soc. 2005, 127, 3516–3526.

(30) Cabeza, J. A.; del Rio, I.; Miguel, D.; Sanchez-Vega, M. G. Chem.

Commun. 2005, 3956–3958.

(31) Hanasaka, F.; Tanabe, Y.; Fujita, K.-I.; Yamaguchi, R. Organo-

metallics 2006, 25, 826–831.

(32) Jazzar, R. F. R.; Macgregor, S. A.; Mahon, M. F.; Richards, S. P.;

Whittlesey, M. K. J. Am. Chem. Soc. 2002, 124, 4944–4945.

(33) McGuinness, D. S.; Saendig, N.; Yates, B. F.; Cavell, K. J. J. Am.

Chem. Soc. 2001, 123, 4029–4040.

(

1

(

12) Garrison, J. C.; Youngs, W. J. Chem. ReV. 2005, 105, 3978–4008.

(

13) Crudden, C. M.; Allen, D. P. Coord. Chem. ReV. 2004, 248, 2247–

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273.

14) Kelly, R. A., III; Clavier, H.; Giudice, S.; Scott, N. M.; Stevens,

(

E. D.; Bordner, J.; Samardjiev, I.; Hoff, C. D.; Cavallo, L.; Nolan, S. P.

Organometallics 2008, 27, 202–210.

(

15) Khramov, D. M.; Lynch, V. M.; Bielawski, C. W. Organometallics

2

007, 26, 6042–6049.

16) Diez-Gonzalez, S.; Nolan, S. P. Coord. Chem. ReV. 2007, 251, 874–

83.

17) Sanderson, M. D.; Kamplain, J. W.; Bielawski, C. W. J. Am. Chem.

Soc. 2006, 128, 16514–16515.

(

8

(

(34) Graham, D. C.; Cavell, K. J.; Yates, B. F. Dalton Trans. 2006,

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(

18) Herrmann, W. A. Angew. Chem., Int. Ed. 2002, 41, 1290–1309.

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Soc. 2006, 128, 16514–16515.

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3974–3979.

(

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5779

Scheme 1. Predicted Reactivity of the NHC-Ag-X Transfer

Agent

NHC-silver(I) halide transfer methods to introduce NHCs into

clusters as it appeared to be the most robust. It was considered

that the reagents for in situ deprotonation of imidazolium salts

to generate a free carbene may be reactive with the cluster,

particularly the hydride containing species.

3

7

Figure 2. [(IMes)AgCl] (Mes ) 2,4,6-trimethylphenyl), an NHC

Osmium N-heterocyclic carbene complexes are rare.

A

transfer complex used in this study, represented as a simple diagram.

survey of those known reveals only a few mononuclear

3

8-40

species.

Osmium cluster-NHC derivatives have only

41-43

recently been reported

as having ruthenium deriva-

The lack of research in this area is surprising since

4

1,44-47

plexes with a singly coordinating anion are neutral, with silver

linearly bound to both the carbene and the halide. These

complexes are an attractive class of molecules with potential

tives.

the role of phosphines in cluster chemistry has been well

documented and thoroughly explored. A key feature of small

clusters containing transition metals is the ability of the metal

sites to react in concert. This has the effect of enhancing the

reactivity over that which can be achieved with single-site metal

1

2

for direct transmetalation of the NHC ligand. This reaction is

accompanied by the thermodynamically favorable formation of

solid silver(I) halide, which typically precipitates during the

course of the reaction (Scheme 1).

4

2

complexes. Following several recent studies showing that

NHC-metal bonds are not inert (see above) and that NHCs

are able to participate in various inter- and intramolecular

reactions, it was anticipated that clusters could provide an ideal

reactive class of molecule to study this reactivity. The chemistry

of NHCs in metal hydride clusters has not been probed, and

this was considered to be of potential interest to catalyst design,

reactivity of hydrogen bound to metals, and NHC activation.

The results of the studies presented herein can be separated

into three sections: (1) NHC-AgCl transfer chemistry, which

does not result in the formation of the anticipated product. This

chemistry highlights the pitfalls of using silver(I) chlorides as

an NHC source. (2) Chemistry in which the NHC is transferred

successfully producing new NHC-cluster complexes. (3)

Reactivity involving the NHC-cluster complexes.

The use of NHC silver(I) halides was considered to attempt

the introduction of an NHC. As they are air- and moisture-

stable allowing for convenient handling, these reagents allow

for an easy starting point into the area of NHC-metal cluster

1

2,51

chemistry.

The NHC transfer complex used in this study is 1,3-bis-(2,4,6-

trimethylphenyl)imidazol-2-ylidene-silver(I) chloride (Figure

4

9

2

). This NHC was chosen as it is simple to prepare, and

examples detailing its activity have been shown with other

systems. Other methods that can be used to introduce carbenes

onto metals, aside from the use of free carbenes, include the

use of imidazole salts, which are deprotonated in situ, the use

of basic groups on the reactive metal center to induce depro-

tonation while freeing a site for coordination, and the use of

3

7

electron rich oleﬁns.

. Unusual NHC-AgCl Transfer Chemistry. Initial studies

1

Results and Discussion

using [(IMes)AgCl] were considered with the simplest trios-

mium cluster species including [Os3(µ-H)2(CO)10] and

[Os3(CO)10(CH3CN)2]. Selection of these molecules was based

upon their unsaturated character and propensity toward facile

ligand introduction under mild temperatures. Also, with the

simple metal framework, it was hoped that it would limit the

formation of multiple products.

The introduction of free NHCs can be technically demanding

because of their highly reactive nature. In order to get around

the reactivity of free carbenes, a carbene transfer agent can be

used. One of the most studied transfer agents of this type is an

9,11,12,48-51

NHC-silver(I) halide complex.

Silver(I) NHC com-

(

37) Arnold, P. L.; Pearson, S. Coord. Chem. ReV. 2007, 251, 596–

09.

38) Mondal, T. K.; Mathur, T.; Slawin, A. M. Z.; Woollins, J. D.; Sinha,

C. J. Organomet. Chem. 2007, 692, 1472–1481.

39) Bolano, T.; Castarlenas, R.; Esteruelas, M. A.; Onate, E. Organo-

metallics 2007, 26, 2037–2041.

40) Castarlenas, R.; Esteruelas, M. A.; Onate, E. Organometallics 2005,

4, 4343–4346.

41) Cabeza, J. A.; del Rio, I.; Miguel, D.; Perez-Carreno, E.; Sanchez-

Vega, M. G. Organometallics 2008, 27, 211–217.

42) Cooke, C. E.; Jennings, M. C.; Pomeroy, R. K.; Clyburne, J. A. C.

Organometallics 2007, 26, 6059–6062.

43) Cooke, C. E.; Ramnial, T.; Jennings, M. C.; Pomeroy, R. K.;

Clyburne, J. A. C. Dalton Trans. 2007, 1755–1758.

44) Ellul, C. E.; Saker, O.; Mahon, M. F.; Apperley, D. C.; Whittlesey,

M. K. Organometallics 2008, 27, 100–108.

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Chem., Int. Ed. 2008, 47, 1920–1922.

46) Ellul, C. E.; Mahon, M. F.; Saker, O.; Whittlesey, M. K. Angew.

Chem., Int. Ed. 2007, 46, 6343–6345.

47) Cabeza, J. A.; del Rio, I.; Martinez-Mendez, L.; Miguel, D. Chem.

Characterization of [Os3(µ-H)(µ-Cl)(CO)9(IMes)] (1).

Scheme 2 outlines the reaction conditions and chemicals used

in the generation of [Os3(µ-H)(µ-Cl)(CO)9(IMes)] (1). Thermal

activation was used to introduce the IMes ligand into the

unsaturated hydride cluster. Recrystallization of (1) was ac-

complished from a concentrated toluene and hexanes solution

at -20 °C.

6

(

2

(

IR solution studies on a hexanes solution of (1) indicated

the presence of eightν(CO) bands and were indicative of a new

complex. For comparison, ﬁve IRν(CO) bands are exhibited for

(

1

13

1

the starting material, [Os3(µ-H)2(CO)10]. H and C{ H} NMR

(

solutions in CD2Cl2 studies on the crystalline solid material (1)

1

conﬁrm the presence of an IMes ligand. The H NMR

(

resonances (6H) for IMes ortho-methyl groups appear at 2.10

ppm and 2.12 ppm. A third resonance in the methyl region (6H)

at 2.36 ppm has been assigned to the para-CH3 group present

(

1

on the IMes ligand. H NMR studies also revealed a single

Eur. J. 2006, 12, 1529–1538.

(

48) Arnold, P. L. Heteroat. Chem. 2002, 13, 534–539.

resonance at -13.3 ppm; the accepted region for bridging

5

2,53

49) Ramnial, T.; Abernethy, C. D.; Spicer, M. D.; McKenzie, I. D.;

hydrides ranges from about 0 ppm to -25 ppm.

This

Gay, I. D.; Clyburne, J. A. C. Inorg. Chem. 2003, 42, 1391–1393.

50) De Fremont, P.; Scott, N. M.; Stevens, E. D.; Ramnial, T.;

(

Lightbody, O. C.; Macdonald, C. L. B.; Clyburne, J. A. C.; Abernethy,

C. D.; Nolan, S. P. Organometallics 2005, 24, 6301–6309.

(51) Lin, I. J. B.; Vasam, C. S. Coord. Chem. ReV. 2007, 251, 642–

670.

5

780 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Scheme 2. Reaction of [Os

3

(µ-H)

2

(CO)10] with [(IMes)AgCl]

(IMes)] (1)

Table 1. Selected Bond Lengths and Angles for

[

Os3(µ-H)(µ-Cl)(CO)9(IMes)] (1)

Producing [Os (µ-H)(µ-Cl)(CO)

3

9

Bond Lengths (Å)

Os(1)-C(1)

Os(1)-Os(2)

Os(1)-Os(3)

Os(2)-Os(3)

Os(1)-Cl

2.121(6)

Os(2)-Cl

Os(1)-H

Os(2)-H

C(1)-N(5)

C(1)-N(2)

2.4566(16)

1.91(4)

1.352(8)

1.371(7)

2.8536(4)

2.8629(4)

2.8414(3)

2.4715(15)

Bond Angles (°)

172.70(17) N(5)-C(1)-Os(1)

59.609(9) O(13)-C(13)-Os(1) 178.9(6)

60.359(9) O(14)-C(14)-Os(1) 173.6(6)

60.032(9) O(21)-C(21)-Os(2) 169.9(6)

C(1)-Os(1)-Os(3)

Os(2)-Os(1)-Os(3)

Os(3)-Os(2)-Os(1)

Os(2)-Os(3)-Os(1)

N(2)-C(1)-Os(1)

N(5)-C(1)-N(2)

126.6(4)

129.8(5)

103.4(5)

O(22)-C(22)-Os(2) 175.3(6)

O(22)-C(22)-Os(2) 178.3(6)

distance of 2.121(6) Å, longer than the 2.056(7) Å for C(1)-Ag

4

9

found in [(IMes)AgCl].

Osmium-osmium bond lengths are all very close in value

ranging from 2.8629(1) Å to 2.8414(3) Å. The metal-metal

bond trans to the carbene is the longest in the complex,

Os(1)-Os(3) 2.8629 Å, compared to 2.814(1) Å in the parent

5

3

cluster. While the bond cis to the carbene, Os(1)-Os(2)

.8414(3) Å, is the shortest in the cluster, this bond also supports

the bridging hydride and chloride atoms, lengthened from

2

resonance integrated to a single hydrogen atom, which indicates

that one of the hydrides had been lost from the osmium cluster.

Resonances are also present for the imidazolium ring protons

and the 2,4,6-trimethylphenyl ring protons in the meta position.

54

5

3

2

.683(1) Å in the parent cluster. As the increase is notable, it

may result from the much larger bridging chloride, which acts

in concert with the strong donating abilities of the IMes ligand

to lengthen all metal-metal bonds in the cluster. The steric

inﬂuence of the bulky 2,4,6-trimethylphenyl substituent on

neighboring carbonyls may also play a role in the lengthening

In order to determine atom connectivity, X-ray crystal-

lographic studies were performed on suitable single crystals of

(

1) grown from a concentrated solution in hexanes and toluene

Figure 3). The structure of (1) shows the anticipated formation

5

of the metal-metal bonds to minimize steric crowding.

of an NHC-osmium carbonyl cluster complex, but not the one

predicted by Scheme 1. The molecule can be viewed as an NHC

complex of the osmium carbonyl cluster [Os3(µ-H)(µ-Cl)(CO)9],

where an NHC occupies a coordination site previously engaged

by a CO ligand in [Os3(µ-H)2(CO)10], and a hydride ligand has

been replaced by a chloride ligand. By mass balance, the

reactants [Os3(µ-H)2(CO)10] and [(IMes)AgCl] are incorporated

into the ﬁnal product with concomitant loss of AgH, which,

presumably, disproportionates to elemental silver and H2.

LSIMS and elemental analysis of (1) gave results consistent

with the proposed structure.

Analysis of the bond angles of the carbonyls bonded to Os(1)

and Os(2) reveal the effect of the bulky NHC ligand. The

O(21)-C(21)-Os(2) angle is 169.9(6)°. It shows the largest

deviation from linearity of all the carbonyls present in the cluster

and is located closest to the NHC ligand. Surrounding M-C-O

angles range from 173.6(6)° to 178.9(6)°. The longest carbonyl

bond length is 1.158(8) Å, and the shortest is 1.127(8) Å for

C(13)-O(13) and C(33)-O(33), respectively.

Interaction of the 2,4,6-trimethylphenyls group with carbonyls

on the osmium to which the NHC is bonded and on the

neighboring metal are most likely the reason why a separate

transmetalation reaction of the free IMes could not be achieved

on [Os3(CO)12]. Attempts were made on [Os3(CO)12] when

activated toward substitution chemically with trimethylamine

N-oxide and also thermally by heating a solution of these

reactants. All such reactions resulted in complete decomposition

with no complexes isolable other than unreacted [Os3(CO)12].

These results mirror those recently found by Cabeza at al. for

the reaction of free IMes wherein it is suggested that the carbene

Selected bond lengths and angles associated with the solid

state structure of (1), shown in Table 1, give a C(1)-Os bond

4

1

is too large to substitute a CO.

A comparable NHC-osmium complex is the species

[

Os3(Me2Im)(CO)11] (Me2Im ) 1,3-dimethylimidazol-2-ylidene);

a methyl-containing carbene derivative was found to have an

osmium-carbene bond length of 2.116(9) Å and was prepared

4

1

by Cabeza et al. The NHC-triruthenium carbonyl complex

of the formula [Ru3(Me2Im)(CO)11] was prepared by a similar

3

0

method. [Os3(Me2Im)(CO)11] and [Ru3(Me2Im)(CO)11] have

(

52) Orpen, A. G. J. Chem. Soc., Dalton Trans. 1980, 2509–2516.

53) Orpen, A. G.; Rivera, A. V.; Bryan, E. G.; Pippard, D.; Sheldrick,

G. M.; Rouse, K. D. J. Chem. Soc., Chem. Commun. 1978, 723–724.

54) Eady, C. R.; Johnson, B. F. G.; Lewis, J.; Malatesta, M. C. J. Chem.

Soc., Dalton Trans. 1978, 1358–1363.

55) Biradha, K.; Hansen, V. M.; Leong, W. K.; Pomeroy, R. K.;

Zaworotko, M. J. J. Cluster Sci. 2000, 11, 285–306.

(

Figure 3. ORTEP view of [Os

Ellipsoids are drawn at the 50% probability level.

3 9

(µ-H)(µ-Cl)(CO) (IMes)] (1).

(

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5781

the carbene attached in an equatorial position, and in (1), IMes

is also attached in an equatorial position.

Scheme 3. Reaction of [Os

Cl] Producing [Os (µ-Cl)(CO)10(µ-Ag(IMes))] (2) and [(IMes-

H)][Os (µ-Cl)(CO)10](µ -Ag)[Os (µ-Cl)(CO)10] (3)

3 3 2

(CO)10(CH CN) ] with [(IMes)Ag-

3

0,41

3

1

3

1

C{ H} NMR solution studies on (1) are consistent with the

3

4

3

1

13

1

asymmetry found in the H NMR spectrum. C{ H} resonances

at 18.6 ppm and 19.0 ppm of approximately equal intensity

appear to be the ortho-methyl carbons. Analysis of the aromatic

region of the C{ H} spectrum reveals two resonances for the

arene carbon atoms at the C-3,5 and C-2,6 positions of the 2,4,6-

trimethylphenyl rings of the IMes ligand, compared to a single

resonance for both in Ar-C-3,5 and Ar-C-2,6 in [(IM-

1

3

1

49

es)AgCl]. Single resonances remain for the other carbon atoms,

and it should be noted that the relative positions of the carbon

resonances have shifted upﬁeld by about 1 ppm. This can be

attributed to the large electron rich center (osmium) to which

IMes is now bound. From looking at the X-ray crystal structure

ORTEP diagram, the asymmetry observed in the NMR spectra

can be attributed to the proximity of the IMes ligand to the

bridging chloride and hydride; the ortho-methyl groups closer

to the chloride than to the hydride experience a different

electronic environment.

In metal carbonyl clusters, the carbonyls generally undergo

two types of ﬂuxional processes: (1) rotation at the metal center

and (2) exchange between metal centers via bridging intermedi-

ates. Therefore, if exchange was occurring among all atoms

equally, a single peak would be observed since every carbonyl

would appear equivalent. As temperature is decreased, the NMR

spectrum will begin to exhibit a collapse and distinct peaks for

reaction of [(IMes)AgCl] with the activated osmium species,

[Os3(CO)10(CH3CN)2], does not proceed via loss of AgCl, but

rather a new type of reactivity is observed for NHC-silver(I)

halide complexes.

1

13

H and C NMR solution studies on the red crystalline solid

material (2) revealed the presence of the IMes ligand with an

ortho-methyl resonance at 1.83 ppm (12H) and a para-methyl

resonance at 2.10 ppm (6H). A single resonance at 6.76 ppm

was assigned to the meta-H of the 2,4,6-trimethylphenyl and a

doublet at 6.89 ppm to the imidazol-H. IR solution studies on

a hexanes solution indicated the presence of seven ν(CO) bands.

5

6-58

each carbonyl can become evident.

13

1

In the acquisition of the C{ H) NMR spectrum, the carbonyl

region was well resolved. The region between 186 ppm to 169

ppm revealed nine resonances of approximate intensity 1 and a

single resonance of intensity 0.5. As there are only nine

carbonyls present in the cluster, it is believed that one of the

resonances corresponds to the carbenic carbon, and it has been

tentatively assigned to the signal at 176.3 ppm on the basis of

its integration, as it is expected that the carbonyls would all

have similar integrations due to similar relaxation rates.

+

Curiously, elemental analysis and LSIMS (M m/z 1299.9) were

consistent with the incorporation of ([(IMes)AgCl] into the

cluster, with concomitant loss of acetonitrile.

In order to determine atom connectivity, we used crystal-

lographic techniques. Single crystals of (2) were grown from

hexanes and the results of the study are shown in Figure 4,

with selected bond lengths and angles shown in Table 2.

As shown in Figure 4, the Ag-C bond is maintained in (2),

but the Ag-Cl bond has been cleaved. Two new Ag-Os

1

3

Furthermore, a carbene C resonance at 169.8 ppm has been

6

reported for the mononuclear complex [(η -p-cymene)OsCl2-

(

IMes)][CF3SO3]; therefore, an assignment at 176.3 ppm is

consistent with the handful of known mononuclear osmium-

3

7,59

NHC complexes.

Conclusive assignment of each carbonyl

interactions were found, leading to an NHC-heterobimetallic

resonance was not made; however, nine resonances conﬁrm that

the carbonyl groups are not ﬂuxional at this temperature.

The preceding data suggest a new type of reactivity of

NHC-silver(I) halides in organometallic cluster systems,

involving the incorporation of not just the IMes carbene

fragment but also the associated halide in hydrogen rich cluster

species.

11,51

complex.

The chloride atom was incorporated into a bridging

position within the cluster such that the complex can be

formulated as [Os3(µ-Cl)(CO)10(µ-Ag(IMes))]. Complex (2) is

structurally related to [Os3(CO)10(µ-PPh2)(µ-AgP(C6H5)3)], which

was prepared in a multistep procedure, but the preparation of

Characterization of [Os3(µ-Cl)(CO)10(µ-Ag(IMes)] (2). The

reaction of the activated cluster [Os3(CO)10(CH3CN)2] with

[

(IMes)AgCl] is shown in Scheme 3. Displacement of aceto-

nitrile ligands leads to the formation of [Os3(µ-Cl)(CO)10(µ-

Ag(IMes))] (2) (49 mg, 35%) and the ionic compound [(IMes-

H)][Os3(µ-Cl)(CO)10](µ4-Ag)[Os3(µ-Cl)(CO)10] (3) (15 mg,

13%). The anticipated precipitation of AgCl(s) was not observed,

and no solid material was formed during the reaction. The

(

56) Crabtree, R. The Organometallic Chemistry of the Transition Metals,

th ed.; John Wiley & Sons, Inc.: Hoboken, NJ, 2005.

57) Dyson, P. J.; McIndoe, J. S. Transition Metal Carbonyl Cluster

Chemistry; Gordon and Breach Science Publishers: Amsterdam, 2000.

58) Johnson, B. F. G.; Benﬁeld, R. E. Transition Metal Clusters; John

Wiley & Sons: Chichester, England, 1980.

59) Castarlenas, R.; Esteruelas, M. A.; Onate, E. Organometallics 2007,

6, 2129–2132.

4

(

Figure 4. ORTEP view of [Os

Ellipsoids are drawn at the 50% probability level.

3

(µ-Cl)(CO)10(µ-Ag(IMes))] (2).

2

5

782 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Table 2. Selected Bond Lengths and Angles for

Table 3. Selected Bond Lengths and Angles for

[

Os3(µ-Cl)(CO)10(µ-Ag(IMes)] (2)

{[Os3(µ-Cl)(CO)10](µ4-Ag)[Os3(µ-Cl)(CO)10]}^-(3)

Bond Lengths (Å)

C(1)-Ag

2.120(12)

2.7983(11)

2.8071(13)

1.348(15)

1.334(16)

Os(1)-Os(2)

Os(1)-Os(3)

Cl-Os(1)

2.8179(8)

2.8435(8)

2.462(3)

2.455(4)

Os(2)-Cl

Os(1)-Cl

Ag-Os(2)

Ag-Os(1)

2.462(7)

2.455(6)

2.9015(10)

2.8597(12)

Os(2)-Os(3)

Os(1)-Os(3)

Os(2)-Os(1)

2.8579(15)

2.8583(14)

2.7805(14)

Os(2)-Ag

Os(1)-Ag

C(1)-N(5)

C(1)-N(2)

Cl-Os(2)

Bond Angles (°)

Os(1)-Cl-Os(2)

68.86(16)

60.89(4)

60.90(4)

58.21(3)

Os(1)-Ag-Os(2)

Os(2)-Os(1)-Ag

Os(3)-Os(1)-Ag

57.71(3)

61.90(3)

109.17(6)

Os(2)-Cl-Os(1)

Os(2)-Ag-C(1)

Os(1)-Ag-C(1)

69.94(10)

150.5(4)

148.8(4)

Os(1)-Ag-Os(2)

N(2)-C(1)-N(5)

Os(2)-Os(3)-Os(1)

60.36(3)

104.9(11)

59.412(19)

60.28(2)

Os(2)-Os(1)-Os(3)

Os(1)-Os(2)-Os(3)

Os(2)-Os(3)-Os(1)

Os(1)-Os(2)-Os(2)

60.305(19) Os(2)-Os(1)-Os(3)

+

LSIMS studies (M 1883.4 m/z), and elemental analysis on the

bulk sample was in accord with this formulation.

this class of bimetallic butterﬂy complexes is best achieved using

the synthetic methodology presented here.

In order to determine if carbonyl groups were ﬂuxional,

NMR solution studies were performed on a sample of (2), which

had been synthesized from [Os3(CO)10(CH3CN)2] prepared with

C carbon monoxide and subjected to the reaction via the

method described in the Experimental Section. The appearance

of six C resonance signals indicates that at 25 °C the carbonyl

6

0

Formation of the imidazolium species suggests the presence

of water in the reaction vessel, despite ﬂame drying of the

apparatus and the use of a dry nitrogen atmosphere. Repeated

experiments to generate compounds (2) and (3) under rigorously

dry conditions resulted in similar isolated yields, thus suggesting

1

3

C

1

3

61

+

that abstraction of H may come from another source, such as

13

CH Cl , which was previously distilled and stored over calcium

2

ligands are not ﬂuxional in solution.

hydride.

To determine if any rearrangement or structural changes could

occur, a sample of (2) was dissolved in hexanes and progres-

sively heated. Analysis of the reaction solution by IR spectros-

copy revealed bands in the ν(CO) region corresponding to the

decomposition product [Os3(CO)12] at temperatures greater than

The structure of (3) is similar to that reported for

+

-

62

[(Ph3P)2N] [{Os3H(CO)10}2Ag] by Raithby et al. The struc-

ture of (3) can be considered as saturated as it contains 48

skeletal electrons required if metal-metal bonds are considered

to be composed of localized two-center two-electron interactions

6

2

between osmium atoms. The Os(1)-Os(2) bond is doubly

9

0 °C. No further study was attempted.

bridged by a one-electron donor (Ag) and a three-electron donor

Characterization of [(IMes-H)][Os3(µ-Cl)(CO)10](µ4-Ag)-

Os3(µ-Cl)(CO)10] (3). The second compound isolated from the

6

2

(

Cl). The bridged Os-Os bonds found in (3) are shorter by

[

about 0.08 Å than the unbridged bonds, which is not short

enough to be consistent with double bond character, typically

reaction of [Os3(CO)10(CH3CN)2] with [(IMes)AgCl] was

1

crystallized as a dark scarlet solid (Scheme 3). H solution NMR

6

3

about 0.15 Å shorter.

on the scarlet crystalline solid material (3) revealed the presence

_o_f_t_h_e_{_I_M_e_s_-_H_}+ ion with a distinct resonance signal at 8.25

The formation of (3) is believed to be the result of an

11

equilibrium occurring in the [(IMes)AgCl] species (Scheme 4).

ppm, which had not been present in (2). IR solution studies on

a dichloromethane solution indicated the presence of six ν(CO)

bands in a distinct pattern markedly different from the other

IMes containing complexes.

Once again, X-ray crystallographic techniques were necessary

to assign the atom connectivity in (3). Scarlet single crystals of

The labile nature of the silver-carbene bond inﬂuences the

formation silver-silver contacts and molecular aggregation to

11

-

generate novel structural motifs. The [AgCl2] ion is believed

to react with two molecules of [Os3(CO)10(CH3CN)2], following

acetonitrile loss such that it leads to its complete incorporation

into (3).

(3) were grown, and the results of the study are shown in Figure

1

3

To further conﬁrm the structure, a C CO-enriched sample

5

. The results clearly show that the solid is composed of discrete

1

3

1

+

of (3) was prepared and subjected to C{ H} NMR solution

{IMes-H} ionsandthenewcomplexanion{[Os3(µ-Cl)(CO)10](µ4-

-

studies. Results of this study revealed the presence of six distinct

Ag)[Os3(µ-Cl)(CO)10]} . Associated bond lengths and angles

1

3

resonances in the carbonyl region. The C NMR spectrum

contains resonances expected for a complex exhibiting the

symmetry and carbonyls associated with (3).

are shown in Table 3.

The {[Os3(µ-Cl)(CO)10](µ4-Ag)[Os3(µ-Cl)(CO)10]}^-anion

was conﬁrmed by the observation of a major ion detected by

To determine if any thermal rearrangement could occur, a

small amount of (3) was heated at progressively higher

temperatures in CH2Cl2 up to 110 °C. Analysis of the reaction

mixture by IR spectroscopy revealed bands in the ν(CO) region

consistent with [Os3(CO)12], suggesting the complex decom-

posed under heating. A dark solid which was not soluble in

CH2Cl2 was also present. No other novel reaction products could

be isolated; further study was not attempted.

Trinuclear-osmium cluster compounds have given unpredict-

able reaction products when treated with [(IMes)AgCl], an NHC

(

60) Ahrens, B.; Cole, J. M.; Hickey, J. P.; Martin, J. N.; Mays, M. J.;

Raithby, P. R.; Teat, S. J.; Woods, A. D. Dalton Trans. 2003, 1389–1395.

(

61) Alex, R. F.; Pomeroy, R. K. Organometallics 1987, 6, 2437–2446.

62) Fajardo, M.; Gomez-Sal, M. P.; Holden, H. D.; Johnson, B. F. G.;

Lewis, J.; McQueen, R. C. S.; Raithby, P. R. J. Organomet. Chem. 1984,

Figure 5. ORTEP view of the anionic component of {[Os

3

(µ-

(µ-Cl)(CO)10]} (3). Ellipsoids are drawn at

the 50% probability level.

2

67, C25-C28.

-

Cl)(CO)10](µ

4

-Ag)[Os

3

(63) Churchill, M. R.; Hollander, F. J.; Hutchinson, J. P. Inorg. Chem.

1977, 16, 2697–2700.

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5783

Scheme 4. Equilibrium in NHC Ag(I) Chloride Species Leading to the Formation of Ion Pairs

4

3

4 4

Scheme 5. Reaction of [Os (µ-H) (CO)12] with [(IMes)AgCl]

transfer reagent. As osmium carbonyl compounds can be

synthesized containing a higher number of metal atoms, it was

Using (CH

3

)

3

NO as a Decarbonylation Agent, Producing

5

7,58,64-66

[Os (µ-H) (CO) (n · IMes)] (4)

4

11

desirable to study their reactions with [(IMes)AgCl].

. Successful NHC Transfer Chemistry. Upon examination

2

of our previous results, it was found that [Os3(µ-H)2(CO)10]

using [(IMes)AgCl] undergoes transmetalation, replacing a

carbonyl ligand with IMes. However, the reaction also resulted

in the replacement of a bridging hydride with a bridging

chloride. In considering what led to this result, we suggest that

the more open (less sterically hindered) metal framework that

resulted from replacing carbonyl groups with hydrides plays a

large role in successful transmetalation. Furthermore, to facilitate

NHC transfer we again introduced a leaving group onto the

cluster and used mild reaction conditions.

Characterization of [Os4(µ-H)4(CO)11(n · IMes)] (4). [Os4(µ-

H)4(CO)12] was activated toward substitution by the use of

67

trimethylamine N-oxide, a convenient decarbonylation reagent.

Reaction of the activated complex with 1,3-bis-(2,4,6-trimeth-

ylphenyl)imidazol-2-ylidene-silver(I) chloride [(IMes)AgCl]

produced a crystalline solid characterized (vide infra) as [Os4(µ-

H)4(CO)11(n · IMes)] (4). This is believed to be the ﬁrst example

of an NHC transfer agent displaying typical transfer reactivity

4

2

toward cluster compound, as previously communicated. The

reaction of [Os4(µ-H)4(CO)12] with [(IMes)AgCl] in dichlo-

romethane at room temperature was accomplished by the

removal of a carbonyl with trimethylamine N-oxide (Scheme

5

). AgCl(s) was seen to precipitate as a gray-white solid.

1

Å. Osmium-osmium distances without hydride ligands were

found to range from 2.826(3) Å to 2.8091(11) Å, clearly

differentiating them from hydride-supporting bonds. Usually,

the positions of the hydride ligands can be inferred reliably from

the positions of the longer metal-metal edges; the result here

H solution NMR studies of (4) in CD2Cl2 revealed the

presence of the IMes ligand in addition to four bridging hydride

ligands, which were found to be ﬂuxional (-20.1 ppm, broad).

1

3

C solution NMR studies conﬁrmed the presence of the IMes

ligand, displaying resonances for the imidazole ring and the

,4,6-trimethylphenyl carbons. IR studies of a CH2Cl2 solution

6

5,69,70

further conﬁrms the usefulness of such correlations.

2

Osmium-hydride positions were inserted into the crystal data

and reﬁned with ﬁnal values found to be 1.67(4) Å.

exhibit nine ν(CO) bands. The composition of (4) was supported

by elemental analysis and LSIMS (M ) m/z 1377.9).

+

To determine the atom connectivity, we used X-ray crystal-

lographic techniques. Single yellow crystals of (4) were grown

in a mixture of CH2Cl2 and hexanes; the ORTEP view is shown

in Figure 6. The study determined that there were two

crystallographically unique but structurally similar molecules

in the unit cell. Table 4 contains selected bond lengths and

angles. The structure of (4) contains an IMes ligand bound to

an osmium cluster, with a single carbon-osmium bond,

C(1)-Os(1), which is 2.05(3) Å long. The other molecule in

the asymmetric unit has a C(6)-Os(5) bond length of 2.20(3)

Å. In the cluster, osmium-osmium bond distances are consistent

with those of clusters containing bridging hydrides; therefore,

the hydrides are placed on the long bonds within the osmium

The M4H4 family of clusters is known to exist in two

predominant geometries: a D2d form and a Cs form (Figure

68,69

core.

Osmium-osmium bonds supporting bridging hydrides

were found to have lengths ranging from 2.990(2) Å to 2.938(2)

(

64) Cotton, F. A.; Wilkinson, G.; Murillo, C. A. AdVanced Inorganic

Chemistry, 6th ed.; Wiley: New York, 1999.

65) Pomeroy, R. K. ComphrehensiVe Organometallic Chemistry II, 2nd

ed.; Elsevier Science Ltd.: Terrytown, NY, 1995; Vol. 7.

66) Shriver, D. F.; Atkins, P. W.; Langford, C. H. Inorganic Chemistry;

W. H. Freeman and Company: New York, 1990.

67) Luh, T. Y. Coord. Chem. ReV. 1984, 60, 255–276.

(

4 4

Figure 6. ORTEP view of [Os (µ-H) (CO)11(n · IMes)] (4). El-

lipsoids are drawn at the 50% probability level. 2,4,6-Trimeth-

ylphenyl hydrogens have been omitted for clarity.

(

5

784 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Table 4. Selected Bond Lengths and Angles for

collapse. Concurrently, three resonances at -19.8 ppm, -20.2

ppm, and -21.3 ppm sharpen. The spectrum at -70 °C contains

three peaks believed to correspond to a Cs conformer in the

[

Os4(µ-H)4(CO)11(n · IMes)] (4)

Bond Lengths (Å)

Os(1)-Os(4)

Os(1)-Os(3)

Os(1)-Os(2)

Os(2)-Os(3)

Os(2)-Os(4)

2.826(3)

2.938(2)

2.988(2)

2.8091(11)

2.9524(11)

Os(3)-Os(4)

Os(1)-C(1)

C(43)-O(43)

C(1)-N(2)

C(1)-N(5)

2.9778(13)

2.05(3)

0.98(3)

1.43(3)

1.44(3)

1:2:1 pattern. Although it is expected that a Cs type arrangement

of hydrides would result in a 1:1:1:1 pattern, it appears either

that there is no signiﬁcant chemical shift difference or that some

measure of hydride motion is still active. The resonances in

the spectra at -80 and -90 °C begin to broaden because of

the insolubility of the compound.

Bond Angles (°)

C(1)-Os(1)-Os(3)

C(1)-Os(1)-Os(2)

C(1)-Os(1)-Os(4)

N(5)-C(1)-N(2)

N(5)-C(1)-Os(1)

117.0(8)

N(2)-C(1)-Os(1)

131.2(16)

62.18(6)

60.96(5)

56.58(5)

Variable temperature studies were performed on a sample of

108.9(11) Os(4)-Os(1)-Os(3)

13

1

169.1(9)

97(3)

131.9(17)

Os(4)-Os(1)-Os(2)

Os(3)-Os(1)-Os(2)

(4) labeled with CO gas. C{ H} NMR solution studies were

performed at 22 and 5 °C. At 22 °C, two sharp resonances are

observed: one at 177.3 ppm integrating to three carbon atoms

and one at 166.6 ppm integrating to two carbon atoms. A third

slightly broadened resonance is observed at 171.8 ppm between

two weak signals, which appear to be collapsing into the

baseline. This signal integrated to four carbon atoms. Of

the collapsing signals seen, the broad signal at 174.6 ppm was

the stronger of the two. At 5 °C, the carbonyl exchange appears

to be slowing, as the central resonance at 171.6 ppm begins to

become asymmetric and collapse into the baseline. This is

accompanied by the growth of two previously weak resonances

at 175.6 ppm and 169.3 ppm. Further cooling would result in

these two resonances becoming sharp, and all ﬂuxional motion

would cease. We conclude that the cessation of ﬂuxional

carbonyl motion is parallel to that of the hydrides.

Table 5. Selected Bond Lengths and Angles for

[

Os4(µ-H)4(CO)10(IMes)2] (5)

Bond Lengths (Å)

Os(1)-C(11)

Os(2)-C(21)

Os(1)-Os(4)

Os(1)-Os(3)

Os(1)-Os(2)

C(11)-N(15)

C(11)-N(12)

C(13)-C(14)

C(14)-N(15)

C(13)-N(12)

2.119(10)

2.120(11)

3.0074(7)

3.0361(6)

3.0532(6)

1.356(14)

1.382(14)

1.319(19)

1.355(15)

1.426(15)

Os(2)-Os(4)

Os(2)-Os(3)

Os(3)-Os(4)

C(4)-O(4)

2.8293(6)

2.9429(6)

2.8014(7)

1.118(14)

1.106(15)

1.342(14)

1.366(15)

1.32(2)

C(7)-O(7)

C(21)-N(25)

C(21)-N(22)

C(23)-C(24)

C(23)-N(22)

C(24)-N(25)

1.340(16)

1.394(16)

Bond Angles (°)

Os(4)-Os(1)-Os(3) 55.228(15) Os(4)-Os(3)-Os(2) 58.955(16)

Os(4)-Os(1)-Os(2) 55.228(15) Os(4)-Os(3)-Os(1) 61.865(16)

Os(3)-Os(1)-Os(2) 57.801(14) Os(2)-Os(3)-Os(1) 61.390(15)

Os(4)-Os(2)-Os(3) 58.026(16) Os(3)-Os(4)-Os(2) 63.019(16)

Os(4)-Os(2)-Os(1) 61.353(15) Os(3)-Os(4)-Os(1) 62.906(16)

Os(3)-Os(2)-Os(1) 60.809(15) Os(2)-Os(4)-Os(2) 62.993(15)

The IMes ligand is oriented cis to two Os-H-Os edges and

trans to an Os-Os bond. Similar orientations are found in

70

[

Os4H4(CO)11P(OMe)3] and its ruthenium analogue.

The results here suggest that successful transfer of the NHC

fragment is dependent upon the steric constraints and the metal

site to which the carbene is intended to be transferred. Careful

choice of the reacting cluster is needed to ensure efﬁcient

transfer.

N(15)-C(11)-N(12) 104.5(9)

N(15)-C(11)-Os(1) 129.3(8)

N(12)-C(11)-Os(1) 126.1(8)

N(25)-C(21)-N(22) 103.0(10)

N(25)-C(21)-Os(2) 128.2(9)

N(25)-C(21)-Os(2) 128.4(9)

7

0,71

Characterization of [Os4(µ-H)4(CO)10(IMes)2] (5). Two NHCs

can be added to [Os4(µ-H)4(CO)12] using the displacement of

two acetonitrile ligands from [Os4(µ-H)4(CO)10(CH3CN)2]. This

complex is formed by the reaction of [Os4(µ-H)4(CO)12] with

trimethylamine N-oxide in a solution of CH3CN. Addition of

two equivalents of [(IMes)AgCl] results in the formation of

[Os4(µ-H)4(CO)10(IMes)2] (5) (74 mg, 45%) (Scheme 6).

1

4).

Of these two, the D2d form is more common. The

energy difference between the geometries is small, a conclusion

that is supported by the observation that the hydride atoms are

ﬂuxional on the NMR time scale at ambient temperature. The

Os4H4 core of molecule (4) has pseudo D2d symmetry, with four

long and two short metal-metal distances commonly found in

the [Os4H4(CO)12]/[Ru4H4(CO)12] family of compounds.

Given the symmetry of the solid-state structure of (4), it would

be expected that if the hydrides were nonﬂuxional, the resulting

pattern in the hydride region of the H NMR spectrum would

be a 2:2 ratio of peaks, as a mirror plane bisecting the molecule

would make for two pairs of equivalent hydrides. Given that

the barrier to hydride motion is small, the result is a broad single

resonance at room temperature. Low-temperature NMR solu-

tion studies were used to determine if the solid state structure

was the same as the rigid solution structure, at temperatures

ranging from 22 to -90 °C. A ﬁgure of this study is provided

in Supporting Information.

70

7

0

[

Os4(µ-H)4(CO)10(CH3CN)2] was generated in situ because

of its reactive nature and its presence conﬁrmed by identiﬁcation

with IR solution studies. The data is consistent with that

1

7

2

previously reported. Heating at 60 °C was found to be

necessary to overcome the energy barrier to adding a second

bulky ligand. Attempts to perform this reaction under ambient

conditions were met with excessive decomposition.

71

A word about the isolation of (5) is in order: After the reaction

to form [Os4(µ-H)4(CO)10(CH3CN)2] with [(IMes)AgCl], the

solvent is removed, and the residue is redisssolved in dichlo-

romethane. After placement on the column, an orange band of

(

5) can be rapidly eluted. If care is not taken, this orange band

At T ) 0 to -20 °C, the hydride motion begins to slow such

that multiple resonances of individual hydrides are evident. The

spectrum at -20 °C reveals resonances that appear consistent

with both conformers. Upon further cooling (T) -30 to -40

will decompose on the column to produce a green material that

can be further separated on the same column. After column

+

chromatography on the recovered solid, LSIMS analysis (M

m/z 1654.0) suggested a tetranuclear cluster containing two IMes

ligands. IR studies of CH2Cl2 solution of (5) indicated the

presence of seven ν(CO) bands. Analysis of the orange crystals

°

C), the broad resonances at 20.6 ppm and 20.8 ppm begin to

(

68) Churchill, M. R.; Hollander, F. J. Inorg. Chem. 1980, 19, 306–

1

of (5) in solution by H NMR spectroscopy solution studies

3

10.

(

69) Churchill, M. R. AdV. Chem. Ser. 1978, 167, 36–60.

70) Wei, C.-Y.; Garlaschelli, L.; Bau, R. J. Organomet. Chem. 1981,

revealed the presence of two IMes ligands and four bridging

hydride ligands. Six H resonances in the methyl region

1

2

13, 63–78.

71) Pomeroy, R. K.; Leong, B. K. In Comprehensive Organometalic

(

Chemistry, III; Crabtree, R. H., Mingos, D. M. P., Eds.; Elsevier Ltd.: New

York, 2007; Vol. 6, pp 873-1044.

(72) Johnson, B. F. G.; Blake, A. J.; Martin, C. M.; Braga, D.; Parisini,

E.; Chen, H. J. Chem. Soc., Dalton Trans. 1994, 2167–2175.

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5785

Table 6. Selected Bond Lengths and Angles for

[

Os4(µ-H)3(µ-Cl)(CO)11(IMes)] (6)

Bond Lengths (Å)

Os(1)-C(1)

Os(1)-Os(3)

Os(1)-Os(4)

Os(1)-Os(2)

Os(2)-Os(3)

Os(3)-Os(4)

C(1)-N(2)

2.11(3)

Os(1)-H(12)

Os(1)-H(13)

Os(1)-H(14)

Os(2)-H(12)

Os(3)-H(13)

Os(4)-H(14)

Os(2)-Cl(1)

Os(4)-Cl(1)

Os(2)-Os(4)

1.990(10)

2.08(9)

1.990(11)

1.989(10)

2.09(9)

1.990(10)

2.474(7)

2.454(8)

3.631

2.9714(14)

3.0463(17)

3.0622(17)

2.8518(17)

2.8319(16)

1.35(4)

4 4

Figure 7. Two predominant geometries for the M H family of

clusters.

C(1)-N(5)

1.41(4)

1.37(4)

N(2)-C(3)

Bond Angles (°)

C(1)-Os(1)-Os(3)

C(1)-Os(1)-Os(4)

Os(3)-Os(1)-Os(4)

C(1)-Os(1)-Os(2)

Os(3)-Os(1)-Os(2)

Os(4)-Os(1)-Os(2)

Os(3)-Os(2)-Os(1)

161.9(8)

117.7(8)

56.13(4)

106.1(7)

56.39(4)

72.94(4)

60.20(4)

Os(4)-Os(3)-Os(2)

Os(4)-Os(3)-Os(1)

Os(2)-Os(3)-Os(1)

N(2)-C(1)-N(5)

N(2)-C(1)-Os(1)

N(5)-C(1)-Os(1)

79.41(4)

63.27(4)

63.41(4)

103(2)

130(2)

125.0(19)

1

73

in the H NMR. The osmium hydride distances were found

to be 1.75(3) Å for Os(1)-H(13) and Os(3)-H(13), and the

remaining were found to be 1.76(3) Å, longer than in (4).

The longest metal-metal bonds in (5) are those with bridging

hydrides, the range being 3.0532(6) Å, for Os(1)-Os(2), to

2.9429(6) Å. Bonds without hydrides were shorter at 2.8293(6)

Å and 2.8014(7) Å. This is not surprising given the steric

interaction between adjacent NHC ligands, interactions which

would be minimized by a lengthening of the metal-metal bond.

Also, hydride supporting bonds are known to be longer in

osmium clusters. Additionally, the increased electron density

on the osmium atoms provides for better orbital overlaps in the

metal-metal bonds.

4 4 2

Figure 8. ORTEP view of [Os (µ-H) (CO)10(IMes) ] (5). Ellipsoids

are drawn at the 50% probability level. 2,4,6-Trimethylphenyl

hydrogens have been omitted for clarity.

13

1

13

The C{ H} and C NMR spectra of (5) were acquired from

1

3

a

CO enriched sample to study the carbonyl region at 25 °C.

The presence of weak signals from 175.2 ppm to 183.5 indicates

ﬂuxional behavior on the NMR time scale. Integration of the

region suggests six carbon atoms, although this is likely

unreliable. Sharp resonances at 164.3 ppm, 171.2 ppm, and

integrating to 6 hydrogen atoms each as well as four individual

hydride signals suggested a lowering of symmetry. The aromatic

region displayed four broad singlets for the 2,4,6-trimethylphe-

nyl ring hydrogens and two sharp resonances assigned to the

imidazole ring hydrogens. All peaks in this region integrated

to two hydrogen atoms, again consistent with two NHC ligands

present.

1

73.0 ppm integrate in a 1:1:2 ratio. These resonances show

coupling to the hydrides and are likely due to the carbonyl

ligands on the osmium atoms to which IMes is bound. Similar

to phosphines derivatives, these carbonyls would likely be

7

4,75

13

1

restricted in motion about those metal centers.

Assignment

C{ H} solution NMR studies conﬁrmed the presence of the

of individual carbonyls was not attempted.

IMes ligands in a low-symmetry orientation. Aromatic carbon

resonances are no longer equivalent for the 2,4,6-trimethylphenyl

ring carbons and display a pattern that is consistent with rotation

about the carbon-nitrogen bond and the osmium-carbon

Synthesis of [Os4(µ-H)3(µ-Cl)(CO)11(IMes)] (6). When [Os4(µ-

H)4(CO)10(IMes)2] (5) was subjected to column chromatography

on 200-mesh silica gel 60, the bright orange complex changed

color to dark green when left on the column for more than 10

min. The collected fractions changed color back to orange

overnight. To determine what products resulted from this

redistribution and hence to understand what process was at work,

the compounds present in the time-lapsed orange solution were

isolated (Scheme 7).

(carbene) bond. Observation and assignment of the two carbene

singlets would be difﬁcult as they are expected to fall in the

4

0,59

same region as the carbonyls.

X-ray crystallographic techniques were used to conﬁrm the

presence of the two NHC ligands and, in particular, to determine

their relative placement in the cluster. The results of the X-ray

study are shown in Figure 8 and in Table 6.

The discovery of [Os4(µ-H)4(CO)12], the parent carbonyl for

the tetranuclear reactions, was perhaps expected as several

substitution reactions involving CH3CN ligands with [Os3(CO)12-X-

The hydride ligands of (5) were not located in the Fourier

maps, but their positions have been assigned as bridging the

long metal-metal bonds previously noted through use of

Orpen’s XHYDEX program (see Experimental Section for

(

CH3CN)X] (X ) 1, 2) result in the formation of the parent

61,76

carbonyl [Os3(CO)12].

The isolated [Os4(µ-H)4(CO)11(IMes)]

5

3

details). XHYDEX predicted that the placement of the H

ligandsacrossthebondsOs(1)-Os(2),Os(1)-Os(3),Os(1)-Os(4),

and Os(2)-Os(3) leads to a Cs type arrangement, which differs

from that found in (4).

(

73) Band, E.; Muetterties, E. L. Chem. ReV. 1978, 78, 639–658.

13

74) Mann, B. E.; Taylor, B. F. C NMR Data for Orgnanometallic

Compounds; Academic Press: New York, 1981.

75) Martin, L. R.; Einstein, F. W. B.; Pomeroy, R. K. Organometallics

988, 7, 294–304.

76) Martin, L. R.; Einstein, F. W. B.; Pomeroy, R. K. J. Am. Chem.

Soc. 1986, 108, 338–340.

(

1

The presence of a second NHC ligand has caused the hydride

ligands to be stationary, as four sharp resonances are observed

(

5

786 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Scheme 6. Reaction of [Os

(IMes)AgCl] to Yield [Os (µ-H)

Complex Green Reaction Mixture

4

(µ-H)

4

(CO)10(CH

3 2

CN) ] with

[

4

(CO)10(IMes)

2

] (5) and the

Figure 9. ORTEP view of [Os

Ellipsoids are drawn at the 50% probability level.

4

(µ-H)

3

(µ-Cl)(CO)11(IMes)] (6).

a

yellow solid, was determined to be [Os4(µ-H)3(µ-

Cl)(CO)11(IMes)] (6). IR studies on a CH2Cl2 solution of (6)

indicated the presence of seven ν(CO) bands.

1

H NMR studies revealed the presence of an IMes ligand as

well as two hydride resonances in a 2:1 intensity ratio, leading

Scheme 7. Complexes Isolated by Column Chromatography of

the Green Mixture: Isolation of [Os (µ-H) (µ-Cl)(CO)11(IMes)]

6)

13

1

to the conclusion that three hydrides were present. The C{ H}

NMR spectrum displayed resonances consistent with that of the

IMes ligand, with the carbene carbon resonance tentatively

assigned at 150.7 ppm. X-ray crystallography revealed the atom

connectivity shown in Figure 9; selected bond lengths and angles

are presented in Table 7. LSIMS analysis agreed with the

formulated structure.

4

3

(

The X-ray diffraction data were weak and not well resolved,

with the structure being reﬁned as an inversion twin and the

crystal suspected to have desolvated. Thus, the solution to the

X-ray data, albeit not stellar, conﬁrms the connectivity. While

the starting material contained a tetrahedral metal skeleton, (6)

has a butterﬂy (nonplanar) arrangement. The X-ray structure

reveals a tetranuclear osmium cluster in which one of the

metal-metal bonds of (5) appears to have been broken,

supplanted by a bridging chloride. Two chemically identical

but crystallographically unique molecules are found in the unit

cell. They differ in the orientation of the 2,4,6-trimethylphenyl

groups, likely an effect of crystal packing.

This 62-electron arachno structure is unusual in that the

precursor (5) and the combination of the IMes ligands and the

hydride ligands appear to have been responsible for breaking

the metal-metal bond. The presence of the trinuclear species

(

1) in the reaction mixture suggests a systematic breaking of

bonds possibly promoted by the silica in the column in the

presence of dichloromethane. It is suggested that the presence

of two IMes ligands creates a crowded environment in which

the Os(1)-Os(2) of (5) is weakened by steric interactions

between the IMes fragments. This has recently been observed

for triruthenium carbonyl clusters, which have been reacted with

an excess of NHC such that they result exclusively in the

4

formation of mononuclear ruthenium bis-NHC complexes.

(

4) was probably formed from trace quantities of [Os4(µ-

The source of the chloride, which replaced one of the hydrides

in (5), is speculated to be abstracted either from AgCl or more

likely from dichloromethane in the presence of silica, possibly

due to the acidity of the column material. Hydride species are

known to be susceptible to attack by chlorinated solvents; this

H)4(CO)11(CH3CN)]. However, the isolation of [Os3(µ-H)(µ-

Cl)(CO)9(IMes)] (1) was completely unexpected since no

trinuclear starting material was present. The presence of (1) was

conﬁrmed by NMR and IR spectroscopy. The fourth compound,

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5787

Table 7. Selected Bond Lengths and Angles for

Scheme 8. Reaction of [Ru (µ-H) (CO) ] with [(IMes)AgCl]

4

12

[

Os5(µ5-C)(µ-H)(CO)14(η-C-η-N-C2H2(Mes)] (8)

Using (CH

3

)

3

NO as a Decarbonylation Agent, Producing the

Bond Length (Å)

Proposed [Ru (µ-H) (CO)11(IMes)] (7)

4

Os(1)-Os(2)

Os(1)-Os(3)

Os(1)-Os(5)

Os(2)-Os(4)

Os(2)-Os(5)

Os(3)-Os(4)

Os(3)-Os(5)

Os(4)-N(5)

Os(1)-C(1)

C(3)-C(4)

2.8691(5)

2.8617(5)

2.9300(5)

2.9286(6)

2.8734(5)

2.9172(5)

2.8802(5)

2.112(7)

2.057(8)

1.348(13)

0.931

Os(1)-C(100)

Os(2)-C(100)

Os(3)-C(100)

Os(4)-C(100)

Os(5)-C(100)

N(2)-C(1)

2.073(9)

1.989(9)

1.994(9)

2.093(9)

2.126(8)

1.375(10)

1.371(11)

1.375(11)

1.393(11)

1.123(11)

1.150(11)

2.340

N(2)-C(3)

N(5)-C(1)

N(5)-C(4)

O(42)-C(42)

O(52)-C(52)

Os(1)-H(3)

Os(5)-H(3)

C(3)-H(31)

C(4)-H(41)

Os(1)-Os(4)

0.937

3.673

2.210

Bond Angle (°)

Os(2)-Os(1)-Os(3)

Os(2)-Os(1)-Os(5)

Os(3)-Os(1)-Os(5)

Os(1)-Os(2)-Os(4)

Os(1)-Os(2)-Os(5)

Os(4)-Os(2)-Os(5)

Os(1)-Os(3)-Os(4)

Os(1)-Os(3)-Os(5)

Os(4)-Os(3)-Os(5)

Os(3)-Os(4)-Os(2)

Os(1)-Os(5)-Os(3)

Os(1)-Os(5)-Os(2)

Os(3)-Os(5)-Os(2)

N(5)-Os(4)-C(43)

Os(4)-N(5)-C(1)

Os(4)-N(5)-C(4)

88.009(16) Os(2)-Os(1)-C(1)

59.392(13) Os(3)-Os(1)-C(1)

59.630(13) Os(5)-Os(1)-C(1)

78.604(14) C(1)-Os(1)-C(10)

61.360(13) C(1)-Os(1)-C(11)

88.628(17) C(1)-Os(1)-C(100)

78.912(14) Os(1)-Os(2)-C(100) 46.3(2)

61.366(13) Os(4)-Os(2)-C(100) 45.6(2)

88.719(14) Os(5)-Os(2)-C(100) 47.7(2)

85.851(16) C(21)-Os(2)-C(100) 116.2(4)

59.005(13) C(22)-Os(2)-C(100) 128.9(4)

59.248(13) C(23)-Os(2)-C(100) 119.1(4)

87.570(14) Os(5)-C(100)-Os(4) 147.8(4)

89.0(2)

87.5(2)

132.7(2)

97.4(4)

97.3(4)

86.2(3)

176.4(4)

125.2(6)

125.7(6)

Os(5)-C(100)-Os(1) 88.5(3)

Os(4)-C(100)-Os(1) 123.7(4)

Os(5)-C(100)-Os(3) 88.7(3)

Os(4)-C(100)-Os(3) 91.1(4)

Os(1)-C(100)-Os(3) 89.4(3)

Os(5)-C(100)-Os(2) 88.5(3)

Os(4)-C(100)-Os(2) 91.7(3)

Os(1)-C(100)-Os(2) 89.9(4)

Os(3)-C(100)-Os(2) 177.1(5)

Os(3)-Os(4)-C(100) 43.1(2)

Os(2)-Os(4)-C(100) 42.8(2)

IR solution studies on (7) reveal seven bands in the ν(CO)

region (Figure 10). The infrared band pattern is similar to that

of [Os4(µ-H)4(CO)11(n · IMes)] (4), suggesting similar symmetry

N(5)-Os(4)-C(100) 83.2(3)

78

and composition. The shift of the bands to lower wavenumbers

relative to the osmium analogue is expected and also observed

for [Ru3(CO)12] vs [Os3(CO)12], which have the same structural

arrangement but a slight shift in IR band pattern in the ν(CO)

C(1)-N(2)-C(3)

C(1)-N(5)-C(4)

N(5)-C(1)-N(2)

N(5)-C(1)-Os(1)

N(2)-C(1)-Os(1)

N(2)-C(3)-C(4)

N(5)-C(4)-C(3)

110.3(8)

109.1(7)

105.3(7)

121.7(6)

133.0(6)

107.6(8)

107.7(8)

7

8-80

region.

Compound (7) was found to decompose at room temperature

even under nitrogen atomosphere to a solid that was insoluble

in dichloromethane. Lowering the temperature appeared to slow

the decomposition as a sample placed in the freezer at -20 °C

was found to be stable for up to one week. The brownish-red

decomposition products could be readily removed by column

chromatography on silica gel without causing structural alter-

ation of (7) as conﬁrmed by IR and NMR studies. Unfortunately,

all attempts to identify these decomposition products were

unsuccessful.

7

is suspected to occur by a radical intermediate. This could

lead to elimination of an IMes fragment as an ionic species

such as [H-IMes]⁺on the column as previously seen in the

formation of (3).

It is suggested that the introduction of sterically demanding

NHC species into transition metal clusters could allow for their

use in the breaking of metal-metal bonds to generate metal-

skeletal arrangements that are difﬁcult to synthesize by other

means. The results described here suggest a role for NHCs

beyond that of a simple ligand in osmium clusters, a role for

which they are already known in main group chemistry.

Characterization of [Ru4(µ-H)4(CO)11(n · IMes)] (7). In the

course of the analysis, [Os4(µ-H)4(CO)11(n · IMes)] (4) was

found to be exceptionally robust. Complexes of ruthenium are

Further support for the formation of [Ru4(µ-H)4(CO)11-

1

(n · IMes)] comes in the form of H NMR data, which revealed

the presence of the IMes ligand and hydride signals. The

broadness of the resonances at -17.76 ppm and 18.02 ppm is

due to ﬂuxional motion of the hydrides.

1

3

1

The C{ H} NMR spectrum was found to have peaks

consistent with one IMes ligand and displayed a pattern similar

to that observed for the osmium derivative. Additional peaks

of lower intensity are believed to be due to impurities. The

5

6,58,64

generally much more labile than those of osmium;

thus,

it was considered that the generation of a ruthenium analogue

of (4) may be difﬁcult under ambient conditions. Consonant

with our results, Ellul et al. and Cabeza et al. have recently

reported interesting results on the reaction of NHCs with the

trinuclear ruthenium carbonyl, [Ru3(CO)12], speciﬁcally on the

activation of the backbone C(3) and C(4) positions and the

1

3

1

C{ H} NMR spectrum of the carbonyl region was acquired

and analyzed. Three resonances were found at 188.6 ppm, 195.0

ppm, and 198.3 ppm in a relative intensity ratio of 3:7:2, though

it is acknowledged that the signal with the integral of 7 may be

inaccurate due to broadening. The pattern and integrals are

similar to those of (4) observed at room temperature, although

30,46

activation of an organic methyl group attached to the NHC.

Reaction of [Ru4(µ-H)4(CO)12] under identical conditions to

those used to generate (4) yields a single orange product (7)

appeared to have the formula [Ru4(µ-H)4(CO)11(n · IMes)] based

upon spectroscopy (Scheme 8).

(78) Stuart, B. H. Infrared Spectroscopy: Fundamentals and Applica-

tions; John C. Wiley & Sons, Inc.: Hoboken, NJ, 2004.

(

79) Rushman, P.; Van Buuren, G. N.; Shiralian, M.; Pomeroy, R. K.

Organometallics 1983, 2, 693–694.

(80) Eady, C. R.; Johnson, B. F. G.; Lewis, J. J. Organomet. Chem.

1973, 57, C84-C86.

(

77) Watson, W. H.; Kandala, S.; Richmond, M. G. J. Chem. Crystallogr.

2

006, 36, 71–75.

5

788 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

2 2

Figure 10. IR spectrum of (7) (left) and (4) (right) in CD Cl , ν(CO) region.

the resonance at 195.0 ppm for the ruthenium derivative is

sharper, indicating a higher rate of ﬂuxional motion. The

resonance at 172.3 ppm is attributed to the carbenic carbon.

Mass spectroscopic analysis was attempted on a solid sample

of (7), but these were inconclusive because of decomposition;

however, elemental analysis was successful and in accord with

the formulation of (7).

number of metal atoms than the starting material. Also, two

tetranuclear clusters were isolated from the reaction mixture.

Infrared studies on a solution of (8) in dichloromethane show

1

eight bands in the ν(CO) region. H solution NMR of the yellow

solid (8) revealed peaks consistent with the IMes ligand.

However, the imidazole protons gave rise to two separate

signals, at 6.31 (1H) and 7.08 ppm (1H), indicating asymmetry

1

3

1

The thermal instability of (7) is most likely the result of

substituent steric strain caused by the bulky mesityl rings. The

weaker metal-metal bonding in ruthenium complexes, as

compared to osmium, creates conditions in which the complex

in the ligand. C{ H} solution NMR studies conﬁrm the

presence of the IMes ligand and also reveal a resonance at 377.9

8

1-83

ppm, the region typical for carbide ligands.

To determine

atomic connectivity, X-ray crystallographic techniques were

employed, and the results of the study are shown in Figure 11

with selected bond lengths and angles in Table 8.

6

4

is more likely to decompose under ambient conditions. It is

worth mentioning that attempted reaction of [Ru4(µ-H)4(CO)12]

and [(IMes)AgCl] at 60 °C in toluene resulted in complete

decomposition of all material to an insoluble black solid. While

many mononuclear ruthenium-NHC complexes are known,

including the second generation Grubbs catalysts, few examples

The study revealed the presence of a pentanuclear osmium

cluster with a previously observed bridged butterﬂy arrangement

of metal atoms and a carbide atom at the center of the framework

8

0,81,83

bonded to all ﬁve metal atoms.

The IMes ligand has

3

7,41,44,46

of ruthenium-NHC cluster species exist.

However, a

undergone decomposition, losing one of its 2,4,6-trimethylphe-

nyl rings, presumably via C-N bond activation and allowing

for coordination to an osmium atom through an imidazole ring

nitrogen. The osmium-carbene bond is intact. The carbide atom

is believed to come from the reaction of two molecules of CO

to form CO2 and C, a known process in carbonyl chemistry,

recently prepared triruthenium carbonyl complex containing an

NHC ligand, 1,3-dimethylimidazol-2-ylidene (Me2Im), was

found to orient itself so as to minimize steric interactions of

the methyl groups with neighboring CO ligands attached to the

30

same ruthenium atom. Given the much larger size of the 2,4,6-

trimethylphenyl groups in (7), it is unlikely that an energetically

favorable conformation exists; their interaction with the CO

groups on neighboring ruthenium atoms likely causes too much

steric strain.

8

4,85

rather than originating from an organic carbon.

This belief

1

3

was conﬁrmed by CO enrichment studies. Other mechanisms

for carbide formation have been suggested including proton-

induced reduction of CO, which given the loss of hydrides from

8

6,87

The properties of ruthenium derivative (7) highlight the

advantages of osmium analogues that offer greater stability. The

isolation of stable osmium species, which may otherwise be

unstable or persist only as postulated intermediates in ruthenium

systems, has the potential to shed light on complex reaction

pathways. Given the importance of ruthenium as a catalyst, the

isolation of osmium species possessing novel bonding motifs

is of great importance to structural transition metal chemistry

and catalytic systems.

(

4), may play a role as well.

Cabeza et al. have recently

communicatedastructurallysimilarcomplexforaruthenium-NHC

(

81) Johnson, B. F. G.; Lewis, J.; Nelson, W. J. H.; Nicholls, J. N.;

Puga, J.; Raithby, P. R.; Rosales, M. J.; Schroeder, M.; Vargas, M. D.

J. Chem. Soc., Dalton Trans. 1983, 2447–2457.

(82) Mason, J. J. Am. Chem. Soc. 1991, 113, 6056–6062.

(83) Eady, C. R.; Johnson, B. F. G.; Lewis, J. J. Chem. Soc., Dalton

Trans. 1975, 2606–2611.

(84) Bailey, P. J.; Duer, M. J.; Johnson, B. F. G.; Lewis, J.; Conole, G.;

McPartlin, M.; Powell, H. R.; Anson, C. E. J. Organomet. Chem. 1990,

3

. Reactivity of an Osmium-NHC Complex. Character-

3

83, 441–461.

85) Anson, C. E.; Bailey, P. J.; Conole, G.; Johnson, B. F. G.; Lewis,

ization of {Os5(µ5-C)(µ-H)(CO)14[η-C-η-NC2H2(Mes)]} (8).

(

Thermolysis of [Os4(µ-H)4(CO)11(n · IMes)] (4) in benzene at

J.; McPartlin, M.; Powell, H. R. J. Chem. Soc., Chem. Commun. 1989, 442–

444.

2

00 °C yields four isolable clusters (Scheme 9). Separation of

(

86) Whitmire, K. H.; Shriver, D. F. J. Am. Chem. Soc. 1981, 103, 6754–

755.

87) Kolis, J. W.; Holt, E. M.; Drezdzon, M.; Whitmire, K. H.; Shriver,

D. F. J. Am. Chem. Soc. 1982, 104, 6134–6135.

these products was achieved by silica column chromatography

with elution in a dichloromethane and hexanes solvent system.

The materials isolated include two clusters that contain a larger

6

(

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5789

Scheme 9. Thermolysis of [Os

C in Benzene, Producing {Os

NC (Mes)]} (8), [Os (µ-H) (µ -C)(CO)15(IMes)] (9), [Os

H) (CO)11(a · IMes)] (10), and {Os (µ-H)(CO)10[(η-C)N(Mes)-

4

(µ-H)

4

(CO)11(n · IMes)] at 200

-C)(µ-H)(CO)14[η-C-η-

(µ-

Table 8. Selected Bond Lengths and Angles for

[

Os6(µ-H)4(µ5-C)(CO)15(IMes)] (9)

°

5

(µ

5

2

H

2

6

4

5

4

Bond Lengths (Å)

Os(1)-Os(3)

Os(1)-Os(2)

Os(2)-Os(4)

Os(2)-Os(5)

Os(2)-Os(3)

Os(4)-Os(6)

Os(4)-Os(5)

Os(3)-Os(6)

Os(3)-Os(4)

Os(5)-Os(6)

C(11)-O(11)

C(53)-O(53)

2.7526(7)

2.8466(8)

2.8369(7)

2.8386(7)

2.8685(7)

2.8705(7)

2.8884(8)

2.8424(7)

2.8862(7)

2.9070(7)

1.170(16)

1.090(18)

Os(1)-C(1)

C(1)-N(5)

C(1)-N(2)

N(2)-C(3)

N(2)-C(81)

C(3)-C(4)

C(4)-N(5)

Os(2)-C

Os(3)-C

Os(4)-C

Os(5)-C

Os(6)-C

2.094(15)

1.350(17)

1.363(17)

1.419(18)

1.422(16)

1.35(2)

1.384(18)

2.039(11)

2.010(13)

2.164(11)

2.011(12)

2.056(11)

4

2

2 2 6 2 3 2 4 4

C H NC H (CH ) (η -C)(η-C)C H (η -C)]} (11)

Bond Angles (°)

Os(4)-Os(2)-Os(5) 61.183(19) Os(1)-Os(3)-Os(4) 68.781(19)

Os(1)-Os(3)-Os(6) 128.92(2)

Os(4)-Os(2)-Os(1) 68.21(2)

Os(5)-Os(2)-Os(1) 128.69(2)

Os(6)-Os(3)-Os(4) 60.138(17)

Os(3)-Os(6)-Os(4) 60.688(17)

Os(3)-Os(6)-Os(5) 88.63(2)

Os(4)-Os(2)-Os(3) 60.774(18) Os(4)-Os(6)-Os(5) 59.987(18)

Os(5)-Os(2)-Os(3) 89.47(2)

Os(3)-C-Os(5)

174.9(7)

90.2(5)

Os(1)-Os(2)-Os(3) 57.583(18) Os(3)-C-Os(2)

Os(3)-Os(1)-Os(2) 61.608(19) Os(5)-C-Os(2)

Os(2)-Os(5)-Os(4) 59.380(18) Os(3)-C-Os(6)

Os(2)-Os(3)-Os(4) 59.072(17) Os(5)-C-Os(6)

89.0(5)

88.7(5)

91.2(5)

Os(2)-Os(4)-Os(6) 91.29(2)

Os(2)-C-Os(6)

170.5(6)

87.4(5)

Os(2)-Os(4)-Os(3) 60.154(18) Os(3)-C-Os(4)

Os(6)-Os(4)-Os(3) 59.174(17) Os(5)-C-Os(4)

87.5(4)

Os(2)-Os(5)-Os(6) 90.50(2)

Os(2)-C-Os(4)

84.9(4)

85.7(4)

Os(4)-Os(5)-Os(6) 59.378(17) Os(6)-C-Os(4)

Os(1)-Os(3)-Os(2) 60.809(19) N(5)-C(1)-N(2)

105.8(13)

126.6(11)

127.3(10)

109.6(12)

106.1(14)

Os(6)-Os(3)-Os(2) 91.22(2)

N(5)-C(1)-Os(1)

Os(2)-Os(4)-Os(5) 59.437(18) N(2)-C(1)-Os(1)

Os(6)-Os(4)-Os(5) 60.634(18) C(1)-N(2)-C(3)

Os(3)-Os(4)-Os(5) 88.15(2)

C(4)-C(3)-N(2)

2.113(7) Å, is consistent with that recently reported by Sinha et

al. for the mononuclear complex [Os(H)(CO)(PPh3)2(PyaiH2)]

(

PyaiH2 ) 2-{3′-(pyridylazo)}imidazole) with an imidazole

38

nitrogen-osmium bond length of 2.101(3) Å.

1

3

1

The C{ H} NMR spectrum of the carbonyl region reveals

eight resonances consistent with the structure of (8). The

appearance of eight resonances of the expected intensity

conﬁrms that the carbonyl structure of this complex is static at

this temperature and that no ﬂuxional motion of the carbonyls

is occurring.

cluster under heating; however, the carbide source is attributed

to carbon abstraction from an organic carbon rather than a CO

molecule. The difference in reactivity maybe due to the

propensity for ruthenium complexes to support activating

bridging ligands under milder conditions in the presence of

NHCs.

Characterization of [Os6(µ-H)4(µ5-C)(CO)15(IMes)] (9).

[Os6(µ-H)4(µ5-C)(CO)15(IMes)] (9) was isolated from the ther-

molysis of [Os4(µ-H)4(CO)11(n · IMes)] (4) (Scheme 9). Infrared

studies on a solution of (9) in dichloromethane showed seven

bands in the ν(CO) region. LSIMS analysis revealed a molecular

ion of m/z 1881.8, suggesting a hexanuclear osmium species.

H solution NMR conﬁrmed the presence of an IMes ligand as

well as showing four hydride resonances.

Suitable crystals of (9) grown from a concentrated solution

in dichloromethane and hexanes were subjected to X-ray

diffraction analysis, and the results are shown in Figure 12 with

selected bond lengths and angles in Table 8.

4

5

1

Complex (8) is a 76-electron system, with the carbene of the

IMes derived ligand acting as bidentate donor, with respect to

66,88

PSEPT.

The carbene donates two electrons, and the nitrogen

center provides one electron as it requires the lone pair for

delocalized imidazole bonding resulting in a formal positive

charge on the atom. Although NMR did not indicate that a

hydride was present, X-ray data and XHYDEX analysis of the

complex suggest a possible bridging site between Os(1) and

Os(5). The Os(1)-Os(5) bond is the longest in the cluster at

The results of the study show a hexanuclear edge-bridged

square pyramidal cluster. The base of the square pyramid

contains a carbide atom similar to (8). Coordination of the

osmium atoms is fulﬁlled by 15 terminal carbonyl ligands. The

positions of the hydride ligands were determined by the XHY-

DEX program with the resulting positions integrated into the

2

.9300(5) Å. In (8), residual electron density was localized

between Os(1) and Os(5).

The metal-carbene bond, Os(1)-C(1), has a length of 2.057(8)

Å, similar to others previously determined. The newly formed

interaction with the imidazole ring nitrogen, Os(4)-N(5) of length

5

2

crystal data. There were two hydride atoms in the basal plane

and two hydrides spanning the edge bridging metal-metal

bonds.

Comparison of the metal-metal bonds with those in (8)

reveals similar bond lengths between the apical atom of the

(

88) Mingos, D. M. P.; Wales, D. J. Introduction to Cluster Chemistry;

Prentice-Hall: Englewood Cliffs, NJ, 1990; pp 195-216; 250-271.

5

790 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Figure 11. ORTEP view of [Os

(Mes)] (8). Ellipsoids are drawn at the 50% probability level.

,4,6-Trimethylphenyl hydrogens have been omitted for clarity.

5 5

(µ -C)(µ-H)(CO)14(η-C-η-N-

2 2

C H

2

Figure 13. ORTEP view of [Os

Ellipsoids are drawn at the 50% probability level.

4

(µ-H)

4

(CO)11(a · IMes)] (10).

of those attached to the apical osmium atom on the square

pyramid. These carbonyls are undergoing ﬂuxional rotation

7

4

about the metal; therefore, their resonance is broad.

Characterization of [Os4(µ-H)4(CO)11(a · IMes)] (10). The

major product of the thermolysis was isolated as a yellow band

eluted following column chromatography. Recrystallization was

achieved from a concentrated solution of dichloromethane and

yielded yellow crystals of (10) similar in appearance to (4).

IR solution studies on (10) in dichloromethane reveal a band

pattern very similar to that of (4). Comparison of the infrared

-

1

spectrum of (4) to that of (10) shows a 1 to 2 cm shift to

lower wavenumbers in (10).

For this study, the resolution of the IR instrument was set to

-1

1

cm to ascertain that the shift was real and not the result of

instrument limitations. These data suggest that the [Os4(µ-

H)4(CO)11] core has been maintained.

1

The H NMR data of (10) were different from that of (4).

For instance, three distinct signals in a 1:2:1 pattern were

6 4 5

Figure 12. ORTEP view of [Os (µ-H) (µ -C)(CO)15(IMes)] (9).

observed in the anticipated region for osmium hydrides rather

5

4

Ellipsoids are drawn at the 50% probability level.

than the single resonance in (4). Also, there are ﬁve signals

with an integration corresponding to 18H for the 2,4,6-

trimethylphenyl CH3 groups, indicating a signiﬁcant lowering

of symmetry for the IMes ligand. Mass spectral data and

elemental analysis indicated that the solid was indeed an isomer

of (4).

pyramid and those that form the base. The average length of

these bonds is 2.871(7) Å in (9) and 2.887(5) Å in (8). The

bonds at the base of the pyramid average to 2.864(7) Å in (9)

compared to 2.899(5) Å in (8).

Examination of the osmium-carbide distances reveal an

average of 2.0549(12) Å for the atoms that lie at the base of

the pyramid and a longer length of 2.164(11) Å to the apical

metal. Similar values were found for (8).

Crystals of (10) were grown from a concentrated CH2Cl2

solution and subjected to X-ray diffraction analysis. The study

determined that there were two crystallographically unique and

structurally similar molecules in the unit cell, and the structures

are shown in Figure 13, with selected bond lengths and angles

shown in Table 9. Indeed, (10) is an isomer of (4). Hydride

positions and potential energies were found using XHYDEX,

and the resulting positions were integrated into the crystal data.

Thus, returning to the IR data, it can be concluded that the

abnormal carbene is slightly more electron-donating than

the normal isomer, as is suggested by the slight lowering of

1

3

1

The C{ H} NMR shows the carbide resonance at 353.8

compared 377.9 ppm in (8). Similar to (8), the carbide acts as

a four-electron donor, exerting no inﬂuence on the periphery

and thereby allowing the four electrons to be added without

the requirement of additional ligands, which would cause steric

8

1

crowding, thus stabilizing (9).

1

3

1

The C{ H} NMR spectrum of the carbonyl region reveals

seven resonances consistent with the structure of (9). The relative

peak intensities are those expected for a molecule containing a

mirror plane bisecting the molecule, and therefore, the hydrides

must be arranged symmetrically. The seven unique carbonyl

resonances are observed in a 2:2:2:2:2:2:3 intensity pattern.

Their complete assignment is not possible with the exception

3

7

the CO stretches.

The structure of (10) contains an IMes ligand bound to an

osmium cluster, with a single carbon-osmium bond, C(3)-Os(1),

which is 2.121(12) Å long compared to the length of Os(1)-C(1)

2.05(8) Å in (4). In the cluster, osmium-osmium bonds

supporting bridging hydrides were found to range from 3.0309(7)

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5791

Figure 14. Space ﬁlling models of the normal (4) (left) and abnormal (9) (right) complexes illustrate the effect of steric crowding at the

location of carbene bonding.

Table 9. Selected Bond Lengths and Angles for

Abnormal carbenes make up a very small percentage of the

known carbene complexes, approximately 2%. However, it

[

Os4(µ-H)4(CO)11(a · IMes)] (9)

37

Bond Lengths (Å)

is believed that number may actually be much higher as many

of the NHC metal complexes generated for use as catalysts and

used in situ lack rigorous characterization of the active species.

More to the point, while ruthenium-NHC complexes are

widely studied well-known catalysts, only recently has the

synthesis of an abnormal carbene cluster complex been reported.

Os(5)-Os(7)

Os(5)-Os(8)

Os(5)-Os(6)

Os(6)-Os(8)

Os(6)-Os(7)

Os(7)-Os(8)

C(6)-N(7)

C(6)-N(10)

N(7)-C(8)

C(8)-C(9)

2.9882(7)

2.9935(7)

3.0142(7)

2.8069(7)

2.9307(7)

2.8166(7)

1.315(17)

1.327(16)

1.431(16)

1.333(19)

C(9)-N(10)

Os(5)-H(5)

Os(5)-H(6)

Os(5)-H(7)

Os(6)-H(5)

Os(6)-H(8)

Os(7)-H(6)

Os(7)-H(8)

Os(8)-H(7)

Os(5)-C(8)

1.367(17)

1.79(4)

1.78(4)

1.79(4)

1.78(4)

1.79(4)

2.095(13)

37

1,3-Di-tert-utylimidazol-2-ylidene (ItBu) reacts with [Ru3(CO)12]

at room temperature to generate a novel abnormal NHC-

4

6

ruthenium complex. This is believed to be the ﬁrst example

of abnormal NHC formation from a free monodentate carbene.

Subsequent heating of this complex results in activation of the

second abnormal (C5) site to afford a triruthenium cluster

Bond Angles (°)

Os(7)-Os(5)-Os(8) 56.181(16) Os(7)-Os(8)-Os(5) 61.815(17)

Os(7)-Os(5)-Os(6) 58.450(16) N(7)-C(6)-N(10)

Os(8)-Os(5)-Os(6) 55.708(16) C(6)-N(7)-C(8)

Os(8)-Os(6)-Os(7) 58.752(17) C(9)-C(8)-N(7)

Os(8)-Os(6)-Os(5) 61.772(16) C(9)-C(8)-Os(5)

Os(7)-Os(6)-Os(5) 60.332(16) N(7)-C(8)-Os(5)

Os(8)-Os(7)-Os(6) 58.430(17) C(8)-C(9)-N(10)

Os(8)-Os(7)-Os(5) 62.003(17) C(6)-N(10)-C(9)

Os(6)-Os(7)-Os(5) 61.218(17) C(8)-Os(5)-Os(7)

Os(6)-Os(8)-Os(7) 62.819(18) C(8)-Os(5)-Os(6)

Os(6)-Os(8)-Os(5) 62.520(17)

108.0(11)

110.9(10)

101.8(11)

128.9(10)

129.3(9)

111.8(12)

107.5(11)

152.3(3)

95.4(3)

4

6

containing an Ru3C2 framework. As previously mentioned,

osmium-NHC complexes are quite rare, and (10) is believed

37

to be the only known abnormal carbene complex of osmium.

Most of the limited knowledge of abnormal carbenes involves

3

7

iridium complexes. From these studies, it has been surmised

that formation of the abnormal carbene may be favored as it

lowers the steric strain at the metal center. On this basis, it is

noted that perhaps the conversion of (4) to (10) is not entirely

unexpected since the steric constraints (steric pressure) are

dramatically decreased upon isomerization from the cramped

pocket around C1 to the more spacious pocket around C4-C3

as shown by the space ﬁlling models (Figure 14).

Å to 2.9235(7) Å compared to a range of 2.990(2) Å to 2.938(2)

Å in (4). Osmium-osmium distances without hydride ligands

were found to range from 2.8177(7) Å to 2.8126(7) Å, clearly

6

5,69,70

differentiating them from hydride-supporting bonds.

The electronic effects seen in other abnormal carbenes mirror

the results found here. Chieﬂy, abnormally bound carbenes are

more electron-donating than normal carbenes on the basis of

Osmium-hydride distances were reﬁned and found to be 1.76(4)

Å. M-C-O angles in (10) range from 172.6(12)° to 179.6(14)°

comparable to those found in (4). Other metrical parameters

such as the tetrahedral metal skeletal bond angles are similar

to those of (4).

1

0,21,22,37

ν(CO) stretching observed in IR spectroscopy.

Also,

abnormal carbene complexes can be identiﬁed by the difference

1

in chemical shifts between the two heterocyclic protons in H

It can be seen that (10) has a Cs type geometry in the solid

state structure, suggested by the 1:2:1 relative intensity of the

NMR spectra (>1 ppm in abnormal carbenes compared with

3

7

1

70,71

<

1 ppm in normal carbenes).

hydride resonances in the H NMR in solution.

A

13

1

C-enriched sample of (10) was prepared and the C{ H}

The imidazole ring hydrogens are not limited to the formation

NMR spectrum acquired at 25 and 5 °C in CD2Cl2. The

observation of broadened resonances indicates that not all

the carbonyls are rigid on the NMR time scale at 25 and 5 °C.

The observation of two sharp resonances at 177.4 ppm and 177.8

of abnormal carbenes. Herrmann et al. have osmylated the NHC

backbone upon treatment of [M(CO)5(C{NMeCH}2] (M ) Cr,

Mo, W) with osmium tetraoxide leading to the formation of

two new osmium-carbon interactions with the imidazole

74,75,89

90

ppm is likely due to axial carbonyl ligands.

The four sharp

carbons.

resonances at 170.4 ppm, 171.0 ppm, 171.5 ppm, and 171.9

ppm are likely due to equatorial carbonyls, which are known

to appear upﬁeld of those of axial carbonyls.

Given the extremely limited number of examples of abnor-

mally bound carbenes that exist for comparison, the direct

conversion of (4) to (10) sheds light on the nature of their

7

4,75,89

(

89) Johnson, B. F. G.; Lewis, J.; Reichert, B. E.; Schorpp, K. T.

(90) Herrmann, W. A.; Roesky, P. W.; Elison, M.; Artus, G.; Oefele,

K. Organometallics 1995, 14, 1085–6.

J. Chem. Soc., Dalton Trans. 1976, 1403–4.

5

792 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

Table 10. Selected Bond Lengths and Angles for

{

Os4(µ-H)(CO)10[(η-C)N(Mes)

2 2

C2H2NC6H2(CH3)2(η -C)(η-C)C4H4(η -C)]} (11)

Bond Lengths (Å)

Os(1)-C(1)

Os(1)-C(69)

Os(2)-C(69)

Os(2)-C(72)

Os(2)-C(71)

Os(4)-C(72)

Os(1)-H(1)

Os(4)-H(1)

C(1)-N(2)

2.090(9)

2.093(10)

2.199(10)

2.241(10)

2.365(10)

2.185(10)

1.74(6)

Os(1)-Os(2)

Os(1)-Os(3)

Os(1)-Os(4)

Os(2)-Os(3)

Os(2)-Os(4)

Os(3)-Os(4)

C(69)-C(71)

C(71)-C(76)

C(71)-C(72)

C(72)-C(73)

C(73)-C(74)

C(74)-C(75)

C(75)-C(76)

2.7608(5)

2.8633(5)

2.9045(5)

2.6324(5)

2.8038(5)

2.8820(5)

1.428(14)

1.409(14)

1.460(13)

1.428(15)

1.348(16)

1.441(15)

1.337(15)

1.74(6)

1.340(12)

1.346(12)

1.393(12)

1.332(14)

1.106(13)

1.149(13)

C(1)-N(5)

N(2)-C(3)

C(3)-C(4)

C(42)-O(42)

C(41)-O(41)

Bond Angles (°)

Os(2)-Os(1)-Os(3) 55.781(12) N(2)-C(1)-N(5)

Os(2)-Os(1)-Os(4) 59.262(13) N(2)-C(1)-Os(1)

Os(3)-Os(1)-Os(4) 59.951(12) N(5)-C(1)-Os(1)

104.3(8)

124.8(7)

130.8(7)

4

Figure 15. ORTEP view of {Os (µ-H)(CO)10[(η-C)N(Mes)-

2

2 2 6 2 3 2 4 4

C H NC H (CH ) (η -C)(η-C)C H (η -C)]} (11). Ellipsoids are

drawn at the 50% probability level. 2,4,6-Trimethylphenyl hydro-

gens have been omitted for clarity.

Os(3)-Os(2)-Os(1) 64.081(13) C(71)-C(69)-C(66) 116.5(8)

Os(3)-Os(2)-Os(4) 63.940(14) C(76)-C(71)-C(69) 124.2(9)

Os(1)-Os(2)-Os(4) 62.922(13) C(76)-C(71)-C(72) 118.3(9)

Os(2)-Os(3)-Os(1) 60.138(13) C(69)-C(71)-C(72) 116.9(9)

Os(2)-Os(3)-Os(4) 60.921(13) C(73)-C(72)-C(71) 116.7(9)

Os(1)-Os(3)-Os(4) 60.734(12) C(74)-C(73)-C(72) 122.2(10)

Os(2)-Os(4)-Os(3) 55.139(13) C(73)-C(74)-C(75) 120.4(10)

Os(2)-Os(4)-Os(1) 57.816(12) C(76)-C(75)-C(74) 119.0(10)

Os(3)-Os(4)-Os(1) 59.315(12) C(75)-C(76)-C(71) 123.2(9)

generation, providing a starting point for the targeted synthesis

of this class of NHCs. We suggest that it is unwise to assume

that NHCs bind via the C1-atom for a complex or a catalyst in

which the NHC is generated in situ (as is the case with

[(IMes)AgCl]). [(IMes)AgCl] is convenient for carbene genera-

tion because it is air- and moisture-stable, but rigorous

characterization of the resulting products is required to be certain

of their identity.

attributed to the coordination of the former methyl group carbon

with the fragment derived from benzene, which places con-

straints on the molecule to allow for favorable orbital overlaps.

During the thermolysis reaction, one of the methyl groups

of the NHC was activated such that all three C-H bonds were

cleaved. The formerly methyl carbon center now bridges two

osmiumsites,formingtwodistinctOs-Cinteractions:Os(1)-C(69)

and Os(2)-C(69). The activation of three C-H bonds on an

NHC supported complex is unprecedented and indicates that

the anticipated enhanced reactivity can be achieved using metal

cluster compounds.

The complex exhibits notable features including the formation

of new bonds to a fragment derived from benzene (solvent).

Three new osmium-carbon bonds result from this coupling:

Os(2)-C(71) 2.365(10) Å, Os(2)-C(72) 2.241(10) Å, and

Os(4)-C(72) 2.185(10) Å. Formation of this coupling could

result from a systematic C-H bond activation of the benzene

ring followed by complexation of an aromatic double bond or

from a double dehydrogenation to produce a benzyne-type

intermediate. However, no mechanistic pathway can be deduced

from the product structure. The ring derived from benzene ex-

hibits bond alternation, the bonds C(75)-C(76) and C(73)-C(74)

have lengths of 1.337(15) Å and 1.348(16) Å, respectively, but

strong shielding of one of the methyl groups on the IMes

fragment indicates that the moiety exhibits a shielding cone,

suggestive of aromaticity within the deformed C6 ring. The bond

between C(71)-C(72) is the longest in the former benzene

molecule and is consistent in length with a single bond. It is

likely somewhat deformed by its coordination to the osmium

atoms and maybe lengthened as a result of electron donation

into the π* orbital of the double bond. The remaining bonds

lengths in the ring between C(71)-C(76), C(75)-C(74), and

C(72)-C(73) appear to be consistent with single bonds.

Finally, a C-C bond formation reaction has occurred,

coupling the dehydrogenated methyl group of IMes and the

dehydrogenated benzene fragment [C(69)-C(71) 1.428(14) Å].

The result of the dehydrogenation and subsequent coupling of

Characterization of {Os4(µ-H)(CO)10[η-C)N(Mes)C2H2NC6-

2

H2(CH3)2(η -C)(η-C)C4H4(η -C)]} (11). The second tetranuclear

metal complex isolated from the thermolysis of (4) was {Os4(µ-

2

H)(CO)10[(η-C)N(Mes)C2H2NC6H2(CH3)2(η -C)(η-C)C4H4(η -

C)]} (11), isolated as a red-brown air-stable solid following

column chromatography (Scheme 9). X-ray diffraction quality

crystals were grown by slow evaporation from a concentrated

mixture of dichloromethane and hexanes under a stream of

nitrogen. The infrared spectrum of (11) shows seven bands in

the ν(CO) region.

1

H NMR studies revealed the presence of ﬁve distinct signals

in the region anticipated for methyl groups with an integration

of 15 H compared to the 18 H expected for the IMes ligand.

Furthermore, in the metal hydride region, a single resonance at

-

19.9 ppm indicated the presence of one unique bridging

54

hydride ligand. Curiously, new peaks were evident in the aryl

1

region of the H spectrum with an ABCD spin system, consistent

with the incorporation of a 1,2-disubstituted benzene ring into

the unit.

An X-ray crystallographic study was completed on a crystal

of (11), and the result is shown in Figure 15, with selected bond

lengths and angles shown in Table 10.

The structure of (11) was unanticipated; however, the

+

elemental analysis and LSIMS (M 1419.7 m/z) are consistent

with the formulation. The complex exhibits an Os4(CO)10 core

with an NHC ligand coordinated to one osmium site with an

Os(1)-C(1) bond interaction that is 2.090(9) Å long. This is

similar in length to those found in the other osmium-NHC

complexes. The single bridging hydride between Os(1) and

Os(4) was located by the use of XHYDEX, the longest

5

2

metal-metal bond in the cluster. Other osmium-osmium

bond lengths range from 2.6324(5) Å to 2.8820(5) Å.

Comparison to the parent cluster (4) reveals that (11) has

closed up considerably. In part, this can be attributed to the

loss of the bridging hydride ligands. Further contraction can be

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5793

Scheme 10. Thermolysis of [Os

8), [Os (µ-H) (µ -C)(CO)15(IMes)] (9), [Os

3 2 2

CH (η -C)(η-C)CHC(CH )C H (η -C)]} (12)

4

(µ-H)

4

(CO)11(n · IMes)] (4) at 200 °C in Toluene, Producing {Os

5

(µ

5

-C)(CO)14[η-C-η-NC

2

H

2

(Mes)]}

6 2

NC H -

(

6

4

5

4

(µ-H) (CO)11(a · IMes)] (10), and {Os (µ-H)(CO)10[(η-C)N(Mes)C

4

2

(

3 2

)

1

the NHC-methyl group and the benzene fragment result in

formal conversion of C(69) from a methyl group to a bridging

carbene. The Os(1)-C(69) 2.093(10) Å bond length is com-

parable to that of Os(1)-C(1). This establishes a six-membered

metallacycle with the NHC group and Os(1). When viewed from

the side, the chain of atoms formed by C(69)-C(71)-C(76)-

C(75)-C(74)-C(73)-C(72) lie in a planar environment, a

condition associated with aromaticity. It is possible that the

delocalized π-system above the planar arrangement is respon-

sible for the shielding experienced by the IMes methyl group.

Mass spectroscopic studies were completed on a sample of

the reaction solvent, which had been vacuum-transferred from

the remaining solid, in an attempt to determine the fate of the

lost hydrogen atoms and to establish if any reduction of benzene

to a 1,4-hexadiene-type molecule had occurred. Our studies only

revealed the presence of benzene. The fate of the hydrogens is

believed to be as released as H2(g), although detection of the

gas from the reaction vessel could not be accomplished because

of the technical difﬁculty associated with this. It is acknowledged

that only volatile compounds would be transferred under vacuum

leaving larger macrocycles and polyphenyl aromatic compounds.

We note this here because of an intriguing experimental result

observed in one experiment. Following elution of the colored

metal complexes, the silica column was ﬂushed with acetone

and the washings collected. Following slow evaporation of this

solution, a small amount (<10 mg) of a clear colorless

crystalline material was recovered. X-ray diffraction studies on

the solid crystalline material identiﬁed it as hexaphenylbenzene,

indicated by the diagnostic bands at 2067 and 2019 cm^-. Some

insoluble black material was noted at the top of the column. It

is important to note that no normal carbene isomer was observed.

The ability of both the normal and abnormal osmium-carbene

species to form (11) suggests that rearrangement is prevalent

at high temperatures. Abnormal and normal species may exist

in an equilibrium at elevated temperatures mediated by revers-

ible intramolecular rearrangement of a hydrogen atom within

the imidazole ring.

Characterization of {Os4(µ-H)(CO)10[η-C)N(Mes)C2H2NC6-

2

H2(CH3)2(η -C)(ηC)CHC(CH3)C2H2(η -C)]} (12). Following

isolation of (11), which contained a fragment derived from the

solvent (benzene), the reaction was repeated in toluene (Scheme

1

0). Using the same preparative method as discussed in the

previous section, clusters (8), (9), and (10) were isolated from

the reaction mixture following chromatography and then identi-

ﬁed by NMR and IR analysis. The new complex, {Os4(µ-

H)(CO)10[(η-C)N(Mes)C2H2NC6H2(CH3)2(η -C)(η-

C)CHC(CH3)C2H2(η -C)]} (12), was isolated as a brown band

from the column, and X-ray quality crystals were grown from

a concentrated solution of hexanes and dichloromethane at -20

°C.

2

Infrared studies displayed a band pattern comparable to that

of (11) with only a slight shift in peak positions. LSIMS analysis

gave a molecular ion of m/z 1433.9, the expected mass of a

1

complex analogous to (11). H solution NMR of (12) revealed

a pattern in the methyl region similar to that of (11), but this

suggested that there were regioisomers present. The highly

shielded resonances at 1.36 ppm and 1.37 ppm, with each signal

integrating to three protons, as well as signals at 2.36 and 2.37

ppm with similar integrations were present. Examination of the

hydride region also showed two resonances. To determine

atomic connectivity an X-ray diffraction was completed, and

the results are shown in Figure 16 with selected bond lengths

and angles presented in Table 11. In toto, the evidence suggests

that the brown solid is composed of two compounds that are

regioisomers of each other formed in a 50:50 ratio.

9

1

clearly suggesting multiple dehydrogenation pathways.

In order to study the reactivity of [Os(µ-H)4(CO)11(a · IMes)]

9), a small sample (45 mg, 0.033 mmol), was thermolyzed at

(

2

00 °C in benzene (20 mL). After 48 h, the reaction mixture

5

4

was then subjected to column chromatography. The isolated

materials corresponded to (11) (23 mg, 0.016 mmol) and a trace

of (9). Infrared spectroscopic evidence for the generation of a

small amount of [Os4(µ-H)4(CO)12] was also obtained, as

(

91) Bart, J. C. J. Acta Crystallopgr., Sect. B 1968, 24, 1277–1287.

5

794 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

2

4 2 2 6 2 3 2 3 2 2

Figure 16. ORTEP view of {Os (µ-H)(CO)10[(η-C)N(Mes)C H NC H (CH ) (η -C)(η-C)CHC(CH )C H (η -C)]} (12). Ellipsoids are drawn

at the 50% probability level. 2,4,6-Trimethylphenyl hydrogens have been omitted for clarity. The methyl group of the toluene was disordered

over two sites. The methyl group was reﬁned isotropically at half-occupancy. Of the four possible sites of addition, only two were observed.

Table 11. Selected Bond Lengths and Angles for {Os4

The complex exhibits an Os4(CO)10 core with an NHC ligand

coordinated to one osmium site with an Os(1)-C(1) bond that

is 2.062(10) Å long, comparable to (11) and similar in length

to those found in other osmium NHC complexes. The hydride

was located by the use of XHYDEX and was found to be

bridging the Os(1)-Os(4) bond of 2.9049(6) Å length. Other

osmium-osmium bond lengths range from 2.6326(7) Å to

2.8569(6) Å.

2

(

µ-H)(CO)10[(η-C)N(Mes)C2H2NC6H2(CH3)2(η -C)(η-

2

C)CHC(CH3)C2H2(η -C)]} (12)

Bond Lengths (Å)

Os(1)-C(1)

Os(1)-C(69)

Os(2)-C(69)

Os(2)-C(72)

Os(2)-C(71)

Os(4)-C(72)

Os(1)-Os(2)

Os(1)-Os(3)

Os(1)-Os(4)

Os(2)-Os(3)

Os(2)-Os(4)

Os(3)-Os(4)

Os(1)-H(1)

Os(4)-H(1)

C(31)-O(31)

2.062(10)

2.094(12)

2.217(11)

2.227(13)

2.377(11)

2.192(14)

2.7597(6)

2.8569(6)

2.9049(6)

2.6326(7)

2.7930(7)

2.8725(7)

1.69(6)

C(69)-C(71)

C(71)-C(72)

C(71)-C(76)

C(72)-C(73)

C(73)-C(74)

C(74)-C(75)

C(74)-C(77)

C(75)-C(76)

C(75)-C(78)

C(1)-N(2)

1.450(16)

1.415(17)

1.465(17)

1.439(17)

1.37(2)

1.45(2)

1.48(3)

1.36(2)

1.54(4)

1.362(13)

1.372(14)

1.430(13)

1.355(17)

1.410(14)

1.123(16)

5

2

An identical series of reactions must have occurred during

thermolysis to form (12) as occurred to form (11) as previously

discussed. The main difference between the formation of (12)

and that of (11) is that the addition is regiospeciﬁc to

accommodate the methyl group of toluene. Unfortunately, no

mechanistic pathway can be deduced from the product structure.

Discrete single-site osmium carbyne complexes have also

been reported, and these were formed from a triple benzylic

dehydrogenation of cymene (4-isopropyltoluene) by an osmium

C(1)-N(5)

N(2)-C(3)

C(3)-C(4)

1.69(6)

1.184(13)

C(4)-N(5)

C(41)-O(41)

Bond Angles (°)

92

amide complex. Finally, metal-bound methyl groups have also

Os(2)-Os(1)-Os(3) 55.869(16) C(71)-C(69)-C(66) 118.0(10)

Os(2)-Os(1)-Os(4) 59.018(17) C(72)-C(71)-C(69) 119.1(11)

Os(3)-Os(1)-Os(4) 59.802(16) C(72)-C(71)-C(76) 122.1(11)

Os(3)-Os(2)-Os(1) 63.934(16) C(69)-C(71)-C(76) 118.2(11)

Os(3)-Os(2)-Os(4) 63.855(19) C(74)-C(73)-C(72) 119.1(16)

Os(1)-Os(2)-Os(4) 63.083(17) C(73)-C(74)-C(75) 121.7(14)

Os(2)-Os(3)-Os(1) 60.198(16) C(73)-C(74)-C(77) 134(2)

Os(2)-Os(3)-Os(4) 60.790(19) C(75)-C(74)-C(77) 104(2)

Os(1)-Os(3)-Os(4) 60.929(16) C(76)-C(75)-C(74) 121.5(14)

Os(2)-Os(4)-Os(3) 55.355(17) C(76)-C(75)-C(78) 132(2)

Os(2)-Os(4)-Os(1) 57.899(15) C(74)-C(75)-C(78) 106.0(19)

Os(3)-Os(4)-Os(1) 59.269(15) C(75)-C(76)-C(71) 116.9(14)

92

been exhaustively dehydrogenated to give carbide complexes.

The remarkable feature present in all these clusters is the

persistent stability of the osmium-carbene bond, which re-

mained intact in some form while decomposition and activation

reactions occurred around the complex. The activation of toluene

and benzene suggests a general reaction for simple aromatic

compounds of this type. Such studies are beneﬁcial to under-

standing NHC decomposition reactions.

N(2)-C(1)-N(5)

N(2)-C(1)-Os(1)

104.1(9)

124.5(8)

N(5)-C(1)-Os(1)

131.0(8)

Conclusions

Recent studies of NHC ligands in cluster systems have yielded

a stunning array of complexes with unique reactivity in a short

period of time. The formation of complexes resulting from

multiple C-H and C-N bond activation has provided valuable

insight into the decomposition of NHC ligands and provides a

stark contrast to the chemistry observed for the ubiquitous

The structure of (12) was analogous to that of (11). However,

the reaction displayed regioselectivity. The two structures

formed in a 1:1 ratio differed only in the orientation of the

toluene-dervied methyl groups. In light of this disorder, the

methyl group of the toluene was reﬁned isotropically with 50%

occupancy. Thus, the disorder was responsible for the appear-

ance of the extra peaks previously observed in the NMR data.

The elemental analysis is consistent with the formulation

presented.

3

0,41,44-46

phosphine ligand.

(

92) Lee, J.-H.; Pink, M.; Caulton, K. G. Organometallics 2006, 25,

802–804.

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5795

The persistence of silver-containing complexes and chloride

substitution in the triosmium complexes treated with [(IM-

es)AgCl] should stand as prime examples of the complex nature

of these reagents as well as the potential pitfalls associated with

NHC transfer reactions. While successful transfer can be

achieved in the case of the tetranuclear osmium-NHC com-

plexes, rigorous characterization is needed to conﬁrm the identity

of the resulting products.

The complex reactivity that has resulted from the thermolysis

of an osmium-NHC complex, notably, the activation of reaction

solvent and the conversion of the carbene to an abnormal mode

of coordination, exemplify the potential of cluster species

containing adjacent metal sites to react in concert. The formation

of higher nuclearity clusters resulting from the addition of

unsaturated metal fragments has revealed interesting results, for

example, the apparent C-N activation of a 2,4,6-trimethylphenyl

group. Finally, the persistence of the osmium-carbene bond

in all of these reaction products is of note, given the existence

of the abnormal carbene derivative seemingly being present at

elevated temperatures.

The thermolysis reaction products, one of which appears to

result from a series of reactions including (a) exhaustive

dehydrogenation of an ortho-methyl group on IMes; (b) partial

dehydrogenation of a solvent molecule; and (c) C-C bond

forming reaction between the two fragments (solvent and cluster)

suggests that NHCs can engage in complex reactions with the

cluster. The activation of benzene and toluene used as solvents

in this system is indicative of the type of complex behavior

that can result from cluster species. Finally, with respect to our

observation of hexaphenylbenzene, dehydrogenation of aromatic

compounds by transition metal complexes can be unpredictable

leading to polyphenyl aromatic compounds. This observation

has implications for the use of clusters as dehydrogenation

catalysts in new organic H2 storage systems Vis- a` -Vis over

dehydrogenation.

Experimental Section

General Procedures and Materials. Care should be exercised

when working with osmium carbonyls becaue of health and

exposure risks. Reactions were carried out under nitrogen using

standard Schlenk techniques. Solvents were puriﬁed by distillation,

with hydrocarbons being distilled over potassium and dichlo-

romethane over calcium hydride. Solvents were stored under an

atmosphere of nitrogen over type 3A molecular sieves. The term,

hexanes refers to a mixture of hexane isomers. High-pressure

reactions were carried out in a 200-mL Parr autoclave. Gases were

supplied by Praxair: nitrogen was pp (prepurifed) grade and dried

93

1

3

2

in a CaH drying tube, and CO was medical grade. CO (99%)

was purchased from Sigma-Aldrich and was used as received.

NMR spectra were collected using a 500-MHz Varian Inova

1

NMR spectrometer (operational frequencies of 499.77 MHz for H

1

3

1

experiments and 125.68 MHz for C{ H} experiments) at SFU

and a Bruker Advance II 600 MHz NMR spectrometer (operational

1

13

1

13

1

frequencies, H, 600.13 MHz; C, 150.90 MHz). H and C{ H}

spectra are referenced to the residual solvent peak of chloroform-d

1

13

1

(

set to 7.260 ppm for H and 77.00 ppm for C{ H} spectra) or

1

2

dichloromethane-d (set to 5.320 ppm for H and 54.00 ppm for

1

3

1

C{ H} spectra).

Infrared spectra were collected using a Bomen MB-100 infrared

spectrometer, and unless otherwise stated, data was collected in

situ using hexanes or dichloromethane as a solvent over a range of

300 cm^-to 1900 cm . The resolution of the IR instrument was

1

-1

2

-1

set to 2 cm or 1 cm , as needed. Mass spectrometry data was

collected using a Kratos Concept double-focusing mass spectrometer

with a LSIMS source. Elemental analyses of the compounds

synthesized was completed at the Simon Fraser University Mi-

croanalytical Laboratory and at Canadian Micro Analytical Service

Ltd.

[

4

Os

3

(µ-H)

2

(CO)10], [Os

(µ-H)

Trimethylamine N-oxide was obtained commer-

3 3 2 4

(CO)10(CH CN) ], [(IMes)AgCl], [Os (µ-

H)

methods.

(CO)12], and [Ru

4

(CO)12] were prepared by literature

4

9,94,95

cially and sublimed overnight at 40 °C.

In addition, the thermolysis reaction led to the generation of trace

amounts of two clusters of higher nuclearity than the reactant. These

clusters have been identiﬁed as {Os5(µ5-C)(µ-H)(CO)14[η-C-η-

NC2H2(Mes)]} and [Os6(µ-H)4(µ5-C)(CO)15(IMes)], both carbide-

containing species. It should be noted that the attempted thermolysis

of [Os4(µ-H)4(CO)10(IMes)2] under similar conditions resulted in

the formation of at least two products, which could not be isolated.

The XHYDEX program of A. G. Orpen (University of Bristol)

was used to predict or conﬁrm the location of the H ligand in some

hydride transition metal clusters.

X-ray Experimental. Crystals of (1)-(6) and (9)-(12) were

mounted on a glass ﬁber. Data were collected on a Nonius Kappa-

CCD area detector diffractometer with COLLECT (Nonius B.V.,

52

1

997-2002). The unit cell parameters were calculated and reﬁned

It is clear from the structure of both (4) and (10) that the

osmium cluster can closely interact with the IMes ligand. This

close approach, during thermolysis and either through agostic

MsH-C interactions or through the loss of a CO ligand, may

result in the activation and dehydrogenation of more than one

C-H bond. This type of multiple C-H bond activation is, to

our knowledge, unprecedented in NHC complexes of single

metal-containing molecules, but multiple C-H activations

appear to be commonplace for metal carbonyl clusters.

from the full data set. Crystal cell reﬁnement and data reduction

were carried out using HKL2000 DENZO-SMN (Otwinowski and

Minor, 1997). The absorption correction was applied using HKL2000

DENZO-SMN (SCALEPACK).

The SHELXTL/PC V6.14 for Windows NT (Sheldrick, G.M.,

2001) suite of programs was used to solve the structure by direct

methods. Subsequent difference Fourier syntheses allowed the

remaining atoms to be located. All of the remaining non-hydrogen

atoms were reﬁned with anisotropic thermal parameters. The

hydrogen atom positions were calculated geometrically and were

included as riding on their respective carbon atoms.

The strong basicity of NHCs increases the electron density

of the metal atoms to which they are attached, and this enhances

the tendency of these metals to become involved in oxidative

addition processes. A combination of these two factors (unsatur-

ated and electron-rich metals) rationalizes the results presented.

A consequence of this reactivity is the multiple oxidative

Crystals of (8) were mounted on a glass ﬁber. Data were collected

on a Bruker Smart Apex II diffractometer equipped with a

Molybdenum X-ray tube (50 kV, 30 mA). Data collection and

reduction were performed using the Bruker Apex II software

package. The unit cell parameters were calculated and reﬁned from

the full data set.

3

addition/cleavage of very strong bonds, such as C-H (sp ) and

2

C-N (sp ).

The results reported herein, when taken with those found by

Cabeza et al. and Whittlesey et al., reveal the utility of ruthenium

and osmium cluster species at generating a wide range of

products with unusual coordination and reactivity when NHCs

(93) Drake, S. R.; Loveday, P. A. Inorg. Synth. 1990, 28, 230–231.

(

94) Kaesz, H. D.; Knox, S. A. R.; Koepke, J. W.; Saillant, R. B.

J. Chem. Soc. D. 1971, 477.

95) Knox, S. A. R.; Koepke, J. W.; rews, M. A.; Kaesz, H. D. J. Am.

Chem. Soc. 1975, 97, 3942–3947.

(

3

0,41,44-46

are engaged as ligands.

5

796 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

The structure was solved via direct methods (sir 92), with

181.4 [s 2C], 181.6 [s, 2C], 185.1 [t, 1C], 185.6 [t, 1C]. MS

96

+

subsequent reﬁnements in CRYSTALS. All non-hydrogen atoms

were reﬁned with anisotropic thermal parameters. The hydrogen

atom positions were calculated geometrically and were reﬁned with

a riding model on their respective carbon atoms. The hydride atom

H(X) was located during reﬁnement (Table 12).

(LSIMS) m/z 1297.8 (M ) (calcd for M ) 1300 (100%), 1299

(83%), 1298 (97%)). Anal. Calc. for C31 26ClAgN : C,

28.63; H, 2.02; N, 2.15. Found: C, 28.40; H, 1.94; N, 2.01.

Compound (3): IR (in CH Cl ) ν(CO): 2095.5 (w), 2085 (m),

037 (br,s), 2032 (sh), 2000 (br,m), 1976(m,sh), 1959(br,m) cm

H

2 3

O10Os

2

-1

2

.

1

Preparation of [Os

tube was added [Os (µ-H)

es)AgCl] (84 mg, 0.19 mmol), CH

3

(µ-H)(µ-Cl)(CO)

(CO)10] (160 mg, 0.19 mmol), [(IM-

Cl (20 mL), and hexanes (30

9

(IMes)] (1). To a Carius

H NMR (500 MHz, 295 K, CD

2

Cl

2 3

) δ 2.14 [s, 12H, ortho-CH ],

3

2

8

1

.41 [s, 6H, para-CH

.25 [s, C -H]. C{ H} NMR (100.61 MHz, 295 K, CD

], 7.16 [s, 4H, meta-H], 7.55 [d, 2H, im-H],

3

+

13

1

2

Cl ) δ

2

mL). The reaction vessel was degassed three times by freeze-pump-

thaw methods and the purple reaction mixture placed in an oil bath

overnight at 60 °C. The following morning, the reaction vessel was

removed from heat and the solution cooled to room temperature.

The reaction vessel contained an orange-yellow solution, and a small

amount of dark solid was present at the bottom of the vessel. The

solvent was removed in Vacuo, the remaining solid dissolved in a

7.7 [s, ortho-CH

3

], 21.5 [s, para-CH

3

], 125.7 [s, NCC], 130.8 [s,

Ar-C-3,5], 134.4 [s, C1], 136.1 [s, Ar-C-2,6],136.9 [s, Ar-C1,],

43.3 [s, Ar-C-4], 175.3 [d, 2C], 179.5 [s, 2C], 183.5 [s, 2C], 185.0

1

+

[

(

t, 1C], 185.3 [s, 2C], 186.0 [t, 1C]. MS (LSIMS) m/z 1883.4 (M )

calcd. M⁺) 1881 (100%), 1882 (92.9%), 1883 (92.3%)). Anal.

Calc. for C41

C, 22.56; H, 1.20; N, 1.40.

Preparation of [Os (µ-H)

(CO) ] (110 mg, 0.1 mmol) in CH Cl (50 mL) was stirred with

2 2 6

H25AgCl N O20Os : C, 22.53; H, 1.15; N, 1.28. Found:

minimum of CH

Elution with CH

band of [Os (µ-H)(µ-Cl)(CO)

2

Cl

2

, and applied to a silica gel column.

/hexanes (3:7 v/v) afforded an orange-yellow

(IMes)], (1). The resulting solid was

4

4 4

(CO)11(n · IMes)] (4). [Os (µ-H) -

2

Cl

2

1

2

3

9

two equivalents of trimethylamine N-oxide (15 mg, 0.2 mmol) in

67,98

recrystallized from hexanes/toluene to afford (1) as orange-red

crystals (41 mg, 19% yield). Other bands were also eluted, and

2 2

CH Cl (20 mL), being added dropwise over 20 min at 22 °C.

Subsequently, [(IMes)AgCl] (47.1 mg, 0.1 mmol) was added. After

stirring for 2 h, the solution had changed color from yellow to dark

orange-brown. The appearance of gray AgCl(s) in the bottom of

the reaction vessel was used to determine reaction completion. The

solvent was removed in Vacuo, the remaining solid dissolved in a

minimum of CH Cl , and applied to a silica gel column. Elution

these corresponded to [Os

determined by infrared spectroscopy.

3 2 3

(µ-H) (CO)10] and [Os (CO)12] as

95,97

Compound (1): IR (in hexanes) ν(CO): 2093 (m), 2051 (s), 2012

vs), 2008 (sh,s), 2002 (s), 1990 (w), 1976 (m), 1971(m), 1939

(

m) cm^-. H NMR (500 MHz, 295 K, CD

1

Cl

], 2.12 [s, 6H, ortho-CH

], 7.05 [s, 2H, meta-H], 7.07 [s, 2H, meta-H], 7.14

) δ -13.28 [s, 1H,

2

µ-H], 2.10 [s, 6H, ortho-CH

H, para-CH

s, 2H, im-H]. C{ H} NMR (125.61 MHz, 298 K, CD

8.6 [s, ortho-CH ], 19.0 [s, ortho-CH ] 21.4 [s, para-CH ], 124.2

s, NCC], 129.7 [s, Ar-C-3,5], 130.4 [s, Ar-C-3,5], 136.3 [s, Ar-

C-2,6], 136.6 [s, Ar-C-2,6] 137.7 [s, Ar-C1,], 140.2 [s, Ar-C-4],

3

], 2.36 [s,

2 2 4

with CH Cl /hexanes (1:4) afforded a orange-yellow band of [Os (µ-

6

[

1

[

3

H) (CO) (n · IMes)] (4), (29 mg, 21%) and a second band corre-

4

11

1

3

1

2

Cl

2

) δ

sponding to [Os (µ-H) (CO) (NMe )], a known compound.

4

11

3

2 2

Compound (4): IR (in CH Cl ) ν(CO): 2094 (sh), 2085 (m), 2064

(

sh), 2053 (vs), 2045 (s), 2023 (s), 1996 (m,br), 1979 (m), 1951

-1

1

w) cm . H NMR (500 MHz, 295 K, CD

2

Cl

2

) δ -20.14 [s,br,

], 7.05 [s,

H, meta-H], 7.09 [d, 2H, im-H]. C{ H} NMR (150.9 MHz, 295

K, CD Cl ) δ 19.4 [s, o-CH ], 21.4 [s, p-CH ], 124.9 [s, NCC],

30.5 [s, Ar-C-3,5], 135.9 [s, Ar-C-2,6], 137.1 [s, Ar-C1,], 141.0

s, Ar-C-4], 166.6 [s, 3C], 168.9 [br,s, 1C], 171.8 [s, 4C], 174.6

1

72.2 (s, 1C), 176.3 [s, carbene-C], 176.9 [s, 1C], 178.1 [s, 1C]

µ-H], 2.12 [s, 12H, ortho-CH

4

3

], 2.35 [s, 6H, para-CH

3

78.2 [s, 1C], 179.8 [s, 1C], 182.8 [s, 1C], 183.9 [s, 1C], 185.4 [s,

13

1

+

C]. MS (LSIMS) m/z 1164.1 (M ) (calcd. for M 1164 (100%)).

Os : C, 30.96; H, 2.17; N, 2.41.

Found: C, 30.93; H, 2.14; N, 2.47.

Preparation of [Os (µ-Cl)(CO)10(µ-Ag(IMes)] (2) and [(IMes-

H)][Os (µ-Cl)(CO)10](µ -Ag)[Os (µ-Cl)(CO)10] (3). To a Schlenk

tube was added [Os (CO)10(CH CN) ] (101 mg, 0.11 mmol),

(IMes)AgCl] (49 mg, 0.11 mmol), and dry CH Cl (30 mL). The

2

3

Anal. Calc. for C30

H25ClN

2

O

9

3

1

[

CD

[

1

3

1

3

br,s, 3C], 177.3 [s, 3C]. C{ H} NMR (150.9 MHz, 278 K,

Cl ) δ 166.6 [s, 2C], 169.3 [br,s, 1C], 171.6 [s, 3C], 175.6

br,s, 2C], 177.3 [s, 3C]. MS (LSIMS) m/z 1377.9 (M ) (calcd for

3

4

3

2

+

3

2

+

[

2

M

28 2 4

) 1378 (100%)). Anal. Calc. for C32H N O11Os C, 27.90, H

resulting reaction mixture was allowed to stir at room temperature

for 2 h. The reaction progress was followed by IR spectroscopy

2

.05, N 2.03. Found C, 27.77, H, 2.10, N 1.93.

Preparation of [Os (µ-H) (CO)10(IMes) ] (5) and [Os

(µ-Cl)(CO)11(IMes)] (6). [Os (µ-H) (CO)12] (110 mg, 0.1

CN (50 mL) was stirred with trimethylamine N-oxide

NO (15 mg, 0.2mmol) in CH CN (20 mL), being added

dropwise over 20 min at 22 °C. The solution was allowed to stir

for approximately until the IR bands for [Os (µ-

H) (CO)10(CH CN) ] were found to dominate. The reaction solution

was ﬁltered through a silica gel plug over a glass frit to remove

any unreacted [Os (µ-H) (CO)12] and (CH NO. Literature prepara-

tions report a 90% yield for this initial step. Because of the

reactive nature of [Os (µ-H) (CO)10(CH CN) ], it was used in situ

4

2

4

(µ-

along with the disappearance of bands in the CO stretching region

H)

3

4

1

associated with [Os

3

(CO)10(CH

3

CN)

2

], speciﬁcally 2023 cm^-. An

mmol) in CH

(

3

orange-red solution resulted. The solvent was removed under

CH

3

)

3

vacuum, the resulting solid dissolved in a minimum of CH

and applied to a silica gel column. Elution with hexanes/CH

afforded two bands. An orange band eluted with CH Cl

1:4). The solvent was removed, and the resulting solid was

recrystallized from hexanes to afford (2) as red crystals (49 mg,

5% yield). A second deep red band eluted with 100% dichlo-

romethane, the solvent was removed, and the resulting solid was

recrystallized from dichloromethane to afford [(IMes-H)][Os (µ-

Cl)(CO)10](µ -Ag)[Os (µ-Cl)(CO)10] (3), as scarlet crystals (15 mg,

3% yield).

Compound (2): IR (in hexanes) ν(CO): 2092 (w), 2036 (vs), 2010

2

Cl

2

,

2

Cl

2

h

4

2

/hexanes

4

3

2

(

4

3 3

)

3

72

4

3

2

3

72

without further characterization or isolation. The reaction mixture

was concentrated down to about 20 mL under vacuum and

transferred to a Carius tube ﬁtted with a stir bar and a Teﬂon valve.

An excess of [(IMes)AgCl] (160 mg, 0.36 mmol) was added to

the reaction vessel, which was then degassed three times by

freeze-pump-thaw methods and placed in an oil bath at 60 °C

overnight. The solution changed color from yellow to dark orange-

brown. The appearance of what appeared to be gray AgCl(s) in the

bottom of the reaction vessel was used to determine reaction

completion. The reaction vessel was cooled to room temperature,

and the mixture was transferred to a Schlenk tube and the remaining

solvent removed under vacuum. The remaining solid was redis-

4

3

1

-

1 1

(

s), 2003 (m), 1981 (w), 1974 (m), 1951 (m) cm . H NMR (400

MHz, 295 K, CDCl ) δ 1.83 [s, 12, ortho-CH ], 2.11 [s, 6H, para-

CH ], 6.76 [s, 4H, meta-H], 6.89 [d, 2H, im-H]. C{ H} NMR

100.61 MHz, 295 K, CD Cl ) δ 17.6 [s, ortho-CH ], 21.4 [s, para-

], 122.6 [d, NCC], 129.8 [s, Ar-C-3,5], 134.9 [s, Ar-C-2,6],

3

1

3

1

3

(

2

3

CH

3

1

35.4 [s, Ar-C1,], 139.7 [s, Ar-C-4], 173.4 [d, 2C], 178.2 [s, 2C],

(

96) Betteridge, P. W.; Carruthers, J. R.; Cooper, R. I.; Watkin, D. J.

J. Appl. Crystallogr. 2003, 36, 1487.

97) Nicholls, J. N.; Vargas, M. D. Inorganic Synthesis; John Wiley &

Sons, Inc.: New York, 1990; Vol. 28, pp 232-235.

(

(98) Li, Y.; Wong, W.-T. J. Cluster Sci. 2001, 12, 595–617.

NHC Complexes of Osmium Clusters

Organometallics, Vol. 27, No. 22, 2008 5797

Table 12

compound

1

2

3

4

5

empirical formula

formula weight(g

C

30

O

H

9

25ClN

3

2

C

31

H

24AgClN

2

O

10Os

3

C

41

H

25AgCl

2

N2O20Os

6

C

32

H

28

N

2

O

11Os

4

C

52

C

H

0.4

52

H

N

4

O

10Os

4

·

Os

· C3.5

H

4

0.8Cl0.80

1209.64

1298.44

2185.60

1377.36

1687.75

-

1

mol

)

temperature (K)

λ(Mo KR) (Å)

crystal system

space group

a(Å)

b (Å)

c (Å)

R (°)

100(2)

150(2)

296(2)

150(2)

0.71073

296(2)

0.71073

triclinic

0.71073

triclinic

0.71073

monoclinic

C2/c

0.71073

monoclinic

P2(1)/n

10.2720(3)

22.8096(5)

24.0937(5)

90

94.1310(10)

90

5630.5(2), 4

1.991

orthorhombic

Pca2(1)

27.9147(5)

16.5133(3)

15.2219(2)

90

j

P1

8.3651(2)

10.8743(3)

20.4975(7)

80.968(2)

81.369(2)

71.935(2)

8.1388(6)

12.5307(9)

18.0480(10)

93.746(4)

97.217(4)

97.567(3)

1803.9(2), 2

2.391

14.5755(8)

21.6908(16)

17.6188(11)

90

106.714(3)

90

ꢀ

(°)

γ (°)

90

3

volume (Å ), Z

1740.42(9), 2

2.308

5334.9(6),4

2.271

7016.7(2), 8

2.608

F (calculated, Mg

-

3

m

)

-

1

µ (mm

F(000)

)

11.056

1122

11.195

1192

14.763

3928

14.500

5008

9.092

3171

crystal size (mm)

0.22 × 0.17 ×

0.28 × 0.15 ×

0.30 × 0.28 ×

0.10 × 0.07 ×

0.20 × 0.10 ×

0

.13

0.08

0.10

0.08

0.05

θ Range (°)

2.58 to 26.37

2.55 to 25.03

-9 to 9, -14 to

14, -21 to 21

17458

2.61 to 23.25

-16 to 16, -24 to

22, -19 to 19

14906

2.57 to 25.03

2.67 to 25.03

index ranges

-10 to 10, -13 to

-33 to33, -19 to

19,-18 to 18

70738

-12 to 12, -27 to

27, -28 to 28

55539

1

3, -25 to 25

number of

21096

reﬂections

collected

number of

independent

reﬂections

7106,[Rint ) 0.054]

6323,[Rint ) 0.095]

3610,[Rint

0.0678]

)

12020,[Rint

0.129]

)

9942,[Rint ) 0.107]

goodness-of-ﬁt on

1.034

0.944

1.007

0.969

1.022

2

F

ﬁnal R indices

R1 ) 0.0344,wR2

R1 ) 0.0540,wR2

) 0.1009

R1 ) 0.0669,wR2

) 0.1610

R1 ) 0.0478,wR2

) 0.0902

R1 ) 0.0495,wR2

) 0.1192

)

0.0822

R indices (all data)

R1 ) 0.0441,wR2

R1 ) 0.1154,wR2

R1 ) 0.1147, wR2

R1 ) 0.0839,wR2

R1 ) 0.0854,wR2

)

0.0856

1.952/-2.350

) 0.1196

1.746/-2.501

) 0.1855

2.457/-2.372

) 0.1011

2.221/-2.559

) 0.1343

1.371/-1.612

largest diff peak

-

3)

hole (e.Å

compound

6

8

9

10

11

12

C

32

H

27ClN

2

O

11Os

4

C

27

H

14

N

2

O14Os

5

C

37

H

28

O

15Os

6

C

32

H

28

N

2

O

11Os

4

C

40

H

33

N

2

O

10Os

4

· C

3

H

7

C

38

H

28

N

2

O10Os

4

·

C0.5HCl

1

0

411.81

50(2)

.71073

1541.41

293

0.71073

monoclinic

P 1 21/n 1

1881.81

150(2)

0.71073

monoclinic

P2(1)/n

1377.36

150(2)

1462.48

150(2)

1475.89

150(2)

0.71073

triclinic

0.71073

triclinic

0.71073

triclinic

orthorhombic

Pna21

j

P1

2

1

3

9

8

2

1

5

0

4.1236(13)

1.6068(6)

1.1183(17)

0

713.1(8),8

.153

1.740

136

17.8450(15)

10.7365(9)

19.3626(16)

90

116.698(4)

90

3314.2(5), 4

3.089

19.167

10.4402(2)

24.0703(6)

17.0669(4)

90

104.1130(10)

90

4159.44(16), 4

3.005

18.326

14.0262(5)

15.9043(6)

16.2286(6)

96.672(2)

96.046(2)

90.049(2)

3575.4(2), 4

2.559

14.229

12.3074(4)

12.6819(4)

14.7598(5)

74.792(2)

87.235(2)

62.317(2)

1961.26(11), 2

2.476

12.976

12.03075(5)

12.7131(7)

15.1134(7)

73.738(2)

86.774(3)

62.890(2)

2013.67(17), 2

2.434

12.704

1350

2728

3360

2504

1342

.40 × 0.30 ×

.03

.62 to 23.26

0.046 × 0.110 ×

0.13 × 0.08 ×

0.35 × 0.25 ×

0.40 × 0.17 ×

0.50 × 0.33 ×

0

0.123

0.05

0.04

0.26

2

-

2.090 to 36.224

-29 to 29, -16 to

17, -30 to -29

117814

2.60 to 25.02

-12 to 12, -28 to

28, -20 to 20

46405

2.71 to 25.03

2.61 to 25.03

2.62 to 25.03

26 to 26, -12 to

-16 to 16, -18 to

18, -19 to 18

37795

-14 to 14, -14 to

15, -17 to 17

22185

-14 to 14, -14 to

15, -17 to 17

24599

1

2, -34 to 34

6

1

9533

2292,[Rint

.086]

.048

)

15182,[Rint

0.073]

)

7334,[Rint ) 0.094]

12034,[Rint ) 0.69]

6620,[Rint ) 0.057]

7057,[Rint )

0

0.0870]

1

0.6095

1.039

1.020

1.040

0.881

R1 ) 0.0736,wR2

0.2010

R1 ) 0.0874,wR2

0.2146

.805/-3.421

R1 ) 0.0314, wR2

R1 ) 0.0464,wR2

R1 ) 0.0672, wR2

R1 ) 0.0513, wR2

R1 ) 0.0411, wR2

)

) 0.0354

) 0.1174

) 0.1781

) 0.1408

) 0.0579

R1 ) 0.0852, wR2

) 0.0576

4.46/-2.42

R1 ) 0.0645, wR2

) 0.1268

2.482/-2.458

R1) 0.0750,

wR2) 0.1878

7.890/-4.527

R1 ) 0.0552, wR2

) 0.1448

6.134/-3.193

R1 ) 0.1143, wR2

) 0.0713

1.334/-1.551

)

3

solved in a minimum amount of CH

column. From multiple preparations, it was found that the orange

band, which appeared on the column, if left alone would change

color to green, decomposing on the column. Application of CH Cl /

2 2

hexanes (3:7) under high nitrogen gas pressure allowed for elution

of the green decomposition material. This was followed by rapid

2

Cl

2

and applied to a silica gel

elution with CH

2

Cl

2

/hexanes (3:2), which afforded an orange band

] (5) (74 mg, 45%).

of [Os (µ-H) (CO)10(IMes)

4

2

Trailing on the column was another green band, which was

collected by elution with dichloromethane. Upon leaving the eluted

green solutions to stir overnight, it was found that they continued

to decompose, resulting in orange solutions the following morning.

5

798 Organometallics, Vol. 27, No. 22, 2008

Cooke et al.

IR studies of this solution revealed a complex set of bands in the

ν(CO) region, suggesting that a mixture of compounds was present

in both samples, which were subsequently combined. The solvent

was removed under vacuum, the residue redissolved in a minimum

[br,s, 2H µ-H], 2.10 [s, 12H, ortho-CH

7.05 [s, 4H, meta-H], 7.09 [d, 2H, im-H]. C{ H} NMR (150.90

MHz, 295 K, CD Cl ) δ 19.0 [s, o-CH ], 21.4 [s, p-CH ], 125.0 [s,

3

], 2.35 [s, 6H, para-CH ],

3

13 1

2

3

NCC], 130.4 [s, Ar-C-3,5], 135.9 [s, Ar-C-2,6], 137.3 [s, Ar-C1,],

172.2 [s, Carbene-C], 188.6 [s, 2C], 195.0 [br,s, 7C], 198.2 [s, 3C].

MS (LSIMS) (calcd for M⁺) 1022 (100%)). Anal. Calc. for

amount of CH

CH Cl /hexanes (1:4) afforded four bands: ﬁrst, [Os

Cl)(CO) (IMes)] (1); second, the previously prepared [Os

H) (CO)11(IMes)] (4); third, [Os (µ-H) (CO)12], the parent osmium

hydride compound; and ﬁnally, the new cluster [Os (µ-H) (µ-

2

Cl

2

, and applied to a silica gel column. Elution with

(µ-H)(µ-

(µ-

2

3

9

4

32 28 2 4

C H N O11Ru C, 37.65, H 2.76, N 2.74. Found C, 37.64, H, 2.77,

4

N 2.63.

Preparation of {Os

Os (µ-H) (µ -C)(CO)15(IMes)] (9), [Os

and {Os (µ-H)(CO)10[η-C)N(Mes)C

4

3

(µ

-C)(µ-H)(CO)14[η-C-ηNC

(µ-H) (CO)11(a · IMes)] (10),

NC

4 4 4 4

C H (η -C)]} (11). [Os (µ-H) (CO)11(n · IMes)] (184 mg, 0.133

2 2

H (Mes)]} (8),

5

Cl)(CO)11(IMes)] (6) (13 mg, 9%). The structures of the previously

characterized compounds (1) and (4) were conﬁrmed by IR and

NMR spectroscopic methods.

[

6

4

5

4

2

4

2

H

2

6 2 3 2

H (CH ) (η -C)(η-C)-

Compound (5): IR (in CH

vs), 1981 (s), 1967 (m), 1943 (m), 1922 (w) cm . H NMR (600

MHz, 298 K, CD Cl ) δ -21.81 [s, 1H µ-H], -21.77 [s, 1H

µ-H],-19.36 [s, 1H µ-H], -18.68 [s, 1H µ-H], 1.95 [s, 6H, ortho-

CH ], 2.03 [s, 6H, ortho-CH ], 2.04 [s, 6H, ortho-CH ], 2.14 [s,

H, ortho-CH ] 2.29 [s, 6H, para-CH ], 2.36 [s, 6H, para-CH ],

.88 [br,s, 2H, meta-H], 6.96 [br,s, 2H, meta-H], 6.99 [s, 2H, im-

2

Cl

2

) ν(CO): 2059 (m), 2034 (s), 2013

mmol) was dissolved in benzene (15 mL) and heated to 200 °C in

a Carius tube degassed three times by freeze-pump-thaw tech-

niques. After 48 h, the solution was cooled and the solvent removed

in Vacuo. The remaining solid was dissolved in a minimum of

-

1 1

(

2

2 2

CH Cl and applied to a silica gel column. IR spectroscopic studies

3

6

3

were performed on the reaction mixtures before and after column

chromatography, and these studies indicated that no rearrangements

H], 7.00 [br,s, 2H, meta-H], 7.01 [s, 2H, im-H], 7.02 [br,s, 2H,

occurred on the column. {Os

(Mes)]} (8) was eluted with CH

yellow band (10 mg, 6%). This was followed by a yellow band of

[Os (µ-H) (CO)11(a · IMes)] (10), (63 mg, 34%). {Os (µ-H)(CO)10[(η-

C)N(Mes)C

eluted with CH

16%). Lastly, elution with CH

red band of [Os (µ-H) (µ -C)(CO)15(IMes)] (9) (11 mg, 7%).

Compound (8): IR (in CH Cl ) ν(CO): 2106 (m), 2078 (s), 2057

5

(µ

5

2 2

H -

1

3

1

meta-H]. C{ H} NMR (150.95 MHz, 290 K, CDCl

2

) δ 18.8 [s,

], 19.7 [s, o-CH ], 21.3 [s,

], 124.1 [s, NCC], 124.6 [s, NCC], 129.5

2

Cl

o-CH

p-CH

3

], 19.2 [s, o-CH

], 21.47 [s, p-CH

3

], 19.5 [s, o-CH

3

4

2

[

s, Ar-C-3,5], 130.1 [s, Ar-C-3,5], 130.2 [s, Ar-C-3,5], 130.5 [s,

2

H

2

NC

Cl

6

H

2

(CH

/hexanes (1:4) as a red-brown band (31 mg,

Cl /hexanes (1:4) gave an orange-

3 2 4 4

) (η -C)(η-C)C H (η -C)]} (11) was

Ar-C-3,5], 135.5 [s, Ar-C-2,6], 136.0 [s, Ar-C-2,6], 136.3 [s, Ar-

C-2,6], 136.4 [s, Ar-C-2,6], 137.2 [s, Ar-C1,], 138.2 [s, Ar-C1,],

2

13

1

2

39.1 [s, Ar-C-4]. 140.4 [s, Ar-C-4]. C{ H} NMR (150.95 MHz,

98 K, CDCl ) δ 164.3 [s, 1C], 171.2 [s, 1C], 173.0 [s, 1C], 173.0

6

4

5

2

[

br,s, 1C], 175.2 [br,s, 1C], 177.9 [br,s], 178.3 [br,s], 178.9 [br, s],

(vs), 2025 (m), 2007 (s), 1990 (br,m), 1975 (sh,w), 1946 (br,w)

1 1

-

1

79.7 [br, s], 181.4 [br,s], 182.6 [br,s], 183.0 [br,s], 183.5 [br,s].

2 2 3

cm . H NMR (500 MHz, 295 K, CD Cl ) δ 1.79 [s, 6H, o-CH ],

1

3

C NMR (150.95 MHz, 298 K, CDCl

C], 171.2 [d, JCH ) 7.7 Hz, 1C], 173.0 [d, JCH ) 4.4 Hz, 1C],

2

) δ 164.3 [d, JCH ) 7.8 Hz,

2.34 [s, 3H, p-CH ], 6.31 [s, 1H, η-N-C-im-H], 6.96 [s, 2H, meta-

3

13 1

1

H], 7.04 [s, 1H, im-H]. C{ H} NMR (150.92 MHz, 295 K,

CD Cl ) δ 18.6 [s, o-CH ], 21.4 [s, p-CH ], 122.1 [s, NCC], 129.5

73.0 [br,s, 1C], 175.2 [br,s, 1C], 177.9 [br,s], 178.3 [br,s], 178.9

2

3

[

br,s], 179.7 [br,s], 181.4 [br,s], 182.6 [br,s], 183.0 [br,s], 183.5

br,s]. MS (LSIMS) m/z 1654.0 (M ) (calcd. M ) 1654 (100%)).

C, 37.76, H 3.17, N 3.39. Found

[s, Ar-C-3,5], 132.4 [s, η-NCC], 136.1 [s, Ar-C-2,6], 137.8 [s,

Ar-C1], 139.5 [s, Ar-C-4]. 160.7 [1C], 170.5 [2C], 172.2 [2C],

173.3 [2C], 173.9 [2C], 174.5 [1C], 176.7 [2C], 181.2 [2C], 377.9

+

52 4 4

Anal. Calc. for C52H N O10Os

+

C, 37.14, H, 3.14, N, 3.12.

[s, µ -C]. MS (LSIMS) m/z 1541.6 (M ) (calcd for M ) 1541

5

(

91.8%), 1542 (100%). Anal. Calc. for C27

H 0.92, N 1.82. Found C, 21.10, H, 0.86, N 1.72.

Compound (9): IR (in CH Cl ) ν(CO): 2088 (m), 2055 (s), 2043

14 2 5

H N O14Os C, 21.04,

2 2

Compound (6): IR (in CH Cl ) ν(CO): 2087 (m), 2060 (vs), 2043

-1 1

(

s), 2022 (s), 1995 (m), 1977 (m), 1948 (br,m) cm . H NMR

600 MHz, 298 K, CD Cl ) δ -18.41 [s, 1H µ-H], -12.64 [s, 2H

], 2.29 [s, 6H, para-CH ], 7.05 [s,

H, meta-H], 7.19 [s, 2H, im-H]. C{ H} NMR (150.95 MHz,

98 K, CD Cl ) δ 18.6 [s, o-CH ], 21.5 [s, p-CH ], 125.8 [s, NCC],

30.5 [s, Ar-C-3,5], 136.3 [s, Ar-C-2,6], 137.7 [s, Ar-C1,], 141.9

2

-

1 1

µ-H], 2.24 [s, 12H, ortho-CH

3

(vs), 2018 (s), 2011 (s), 1996 (m), 1947 (br,w) cm . H NMR

1

3

1

4

2

1

(600 MHz, 295 K, CD Cl ) δ -9.85 [s, 1H, µ-H], -9.84 [s, 1H,

2

3

µ-H], -9.80 [s, 1H, µ-H], -9.79 [s, 1H, µ-H], 2.14 [s, 3H, o-CH₃],

2.22 [s, 3H, o-CH ], 2.26 [s, 3H, o-CH ], 2.36 [s, 3H, o-CH ], 2.40

3

[

J

1

s, Ar-C-4]. 150.7 [t, carbene-C], 171.1 [dd, JCH(cis) ) 2.9 Hz,

CH(trans) ) 11.3 Hz 2C], 174.6 [s, 2C], 178.0 [s, 2C], 178.8 [br,s,

C], 180.5 [d, JC ₊H (trans) ) 11.6 Hz, 2C], 182.3 [s, 2C]. MS (LSIMS)

3 3

[s, 3H, p-CH ] 2.50 [s, 3H, p-CH ], 7.13 [s, 1H, meta-H], 7.22 [s,

1H, meta-H], 7.13 [s, 1H, meta-H], 7.13 [s, 1H, meta-H], 7.25 [d,

J_H_H) 2 Hz, 1H, im-H], 7.37 [d, J ) 2 Hz, 1H, im-H], 7.30 [s,

HH

+

13

1

m/z 1412.00 (M ) (calcd. M ) 1412 (100%)). Anal. Calc. for

C, 27.22, H 1.93, N 1.98. Found C, 30.57, H, 2.41,

2 2

1H, meta-H]. C{ H} NMR (150.92 MHz, 295 K, CD Cl ) δ 18.8

C

32

H

27

N

2

O

10Os

4

3 3 3 3

[s, o-CH ], 19.2 [s, o-CH ], 19.4 [s, o-CH ], 19.8 [s, o-CH ], 21.5

N, 1.95.

Preparation of [Ru

0.140 mg, 0.2 mmol) in CH

equivalents of trimethylamine N-oxide (28 mg, 0.4 mmol) in CH

3 3

[s, p-CH ], 21.6 [s, p-CH ], 124.7 [s, NCC], 125.8 [s, NCC], 129.0

4

(µ-H)

4

(CO)11(IMes)] (7). [Ru

Cl (30 mL) was stirred with two

Cl

20 mL), being added dropwise over 20 min at 22 °C. Subse-

4

(µ-H)

4

(CO)12

]

[s, Ar-C-3], 129.2 [s, Ar-C-3], 130.5 [s, Ar-C-5], 130.7 [s, Ar-C-

5], 136.2 [s, Ar-C-2], 136.3 [s, Ar-C-2], 136.6 [s, Ar-C-6], 136.6

[s, Ar-C-1], 137.0 [s, Ar-C-4], 137.5 [s, Ar-C-1], 141.6 [s, Ar-C-

4], 142.0 [s, Ar-C-4], 171.4 [NCN], 173.5 [3C], 173.9 [2C], 174.1

[2C], 178.5 [2C], 178.6 [2C], 185.2 [2C], 186.6 [2C], 353.8 [s,

(

2

9

8

(

quently, [(IMes)AgCl] (47.1 mg, 0.1mmol) was added. After stirring

for 1 h, the solution had changed color from yellow to dark orange-

brown. The appearance of a ﬁne gray powder believed to be AgCl(s)

in the bottom of the reaction vessel was used to determine reaction

completion. The solvent was removed in Vacuo, the remaining solid

+

µ -C]. MS (LSIMS) m/z 1881.8 (M ) (calcd for M ) 1882

5

(100%). Anal. Calc. for C H N O Os C, 23.61, H 1.50, N 1.49.

3

7

28

2

15

6

Found C, 23.32, H, 1.27, N 1.39.

Compound (10): IR (in CH Cl

038 (s), 2021 (s), 1995 (m), 1985 (m), 1977 (m), 1942 (sh,w)

2

) ν(CO): 2084 (m), 2052 (vs),

dissolved in a minimum volume of CH

gel column. Elution with CH Cl /hexanes (1:4) afforded a single

orange-yellow band of [Ru (µ-H) (CO)11(IMes)] (7), (65 mg, 34%).

2 2

Cl , and applied to a silica

2

-1 1

2

cm . H NMR (600 MHz, 295 K, CD

2

Cl

19.85 [s,2H, µ-H], -19.85 [s,1H, µ-H], 2.07 [s, 3H, o-CH

s, 3H, o-CH ], 2.11 [s, 3H, o-CH ], 2.2 [s,br, 3H, o-CH ], 2.2 [s,br,

], 6.63 [s, 1H, meta-H], 6.68 [s, 2H, meta-H], 7.04 [s,

2

) δ -20.50 [s,1H,],

4

-

3

], 2.09

When left at ambient temperature in air or nitrogen for extended

periods of time, the complex decomposed.

[

3

6

H, p-CH

3

Compound (7): IR (in hexanes) ν(CO): 2083 (m), 2063(w), 2048

1H, im-H], 7.07 [s, 1H, im-H], 7.09 [s, 2H, im-H], 7.95 [s, 2H,

vs), 2028 (s), 2002 (m), 1986 (m), 1967 (br,w), 1954 (w) cm^-

1

.

C(1)-H], 7.99 [s, 1H, C(1)-H]. C{ H} NMR (150.92 MHz, 298

K, CD Cl ) δ 17.3 [s, o-CH ], 17.4 [s, o-CH ], 18.3 [s, o-CH ],

13 1

(

1

2 2

H NMR (600 MHz, 295 K, CD Cl ) δ -18.0 [br,s, 2H µ-H], -17.8

2

3

NHC Complexes of Osmium Clusters

1.4 [s, p-CH ], 21.5 [s, p-CH ], 126.0 [s, NCa · NHC · C], 129.9

Organometallics, Vol. 27, No. 22, 2008 5799

2

3

by a yellow band of [Os

12%). {Os (µ-H)(CO)10[(η-C)N(Mes)C

C)CHC(CH )C H (η -C)]} (12) was eluted with CH Cl /hexanes

4 4

(µ-H) (CO)11(a · IMes)] (10), (13 mg,

2

[

s, Ar-C-3,5], 130.1 [s, Ar-C-3,5], 130.2 [s, Ar-C-3,5], 131.4

s, Ar-C-3,5], 134.1 [s, Ar-C-2,6], 134.2 [s, Ar-C-2,6], 134.4

s, Ar-C-2,6], 134.7 [s, Ar-C-2,6] 135.0 [s, Ar-C1], 136.0 [s,

4

2 2 6 2 3 2

H NC H (CH ) (η -C)(η-

2

3

2

(1:4) as a red-brown band (25 mg, 22%). Lastly, elution with

Ar-C1,], 141.4 [s, Ar-C-4], 141.8 [s, NCCHN], 141.9 [s, Ar-C-

4

1

[

CH Cl /hexanes (1:4) gave an orange-red band of Os (µ-H) (µ -

2

6

4

5

], 167.0 [br,s, 1C], 169.6 [sh, 1C], 170.4 [s, 1C], 171.0 [s, 1C],

71.5 [s, 1C], 171.9 [s, 1C], 176.8 [br,s, 1C], 177.4 [s, 2C], 177.8

15

C)(CO) (IMes)] (9), (5 mg, 5%).

Compound (12): IR (in CH

s), 1995 (s), 1979 (m), 1968 (m), 1915 (br,w) cm . H NMR

600 MHz, 295 K, CD Cl ) δ -19.98 [s, µ-H], -19.96 [s, µ-H],

.32 [s, 3H, H57], 1.33 [s, 3H, H57], 2.09 [s, 3H, H78], 2.19 [s,

2 2

Cl ) ν(CO): 2075 (s), 2043 (vs), 2012

1

3

1

s, 1C]. C{ H} NMR (150.92 MHz, 273 K, CD

2

Cl

2

) δ 168.6

-1 1

(

1

3

2

[

br,s, 1C], 169.8 [br,s, 1C], 170.3 [s, 1C], 171.0 [s, 1C], 171.4 [s,

2

1

C], 171.8 [s, 1C], 173.5 [br,s, 1C], 176.0 [br,s, 1C], 177.4 [s, 2C],

+

177.8 [s, 1C], 180.3 [br,s, 1C]. MS (LSIMS) m/z 1377.9 (M ) (calcd

H, H58], 2.29 [s, 3H, H67], 2.32 [s, 3H, H59], 2.33 [s, 3H, H59],

.41 [s, 3H, H77], 2.59 [s, 3H, H68], 6.73 [d, JHH ) 8.5 Hz, 1H,

H74A], 6.88 [s, H, H55A] 6.90 [s, 1H, H53A], 6.91 [s, 1H, H53A],

7.02 [s, 2H, H63A], 7.05 [d, JHH ) 8.9 Hz, H75A], 7.10 [ s, 2H,

H65A], 7.17 [d, JHH ) 5.1 Hz, H4A ], 7.26 [s, 1H, H76A], 7.48

+

for M ) 1378 (100%)). Anal. Calc. for C32

28 2 4

H N O11Os C, 27.90,

H 2.05, N 2.03. Found C, 27.77, H, 1.94, N 1.79.

Compound (11): IR (in CH Cl ) ν(CO): 2076 (s), 2044 (vs), 2013

s), 1996 (s), 1980 (m), 1969 (m), 1916 (br,w) cm . H NMR

600 MHz, 295 K, CD Cl ) δ -19.95 [s, µ-H], 1.32 [s, 3H, H57],

.20 [s, 3H, H59], 2.30 [s, 3H, H58], 2.33 [s, 3H, H67], 2.59 [s,

H, H68], 6.86 [m, 1H, H75A], 6.88 [s, 1H, H53A], 6.92 [s, 1H,

2

-

1 1

(

2

[

d, JHH ) 8.8 Hz, H76A], 7.64 [s, 1H, H73A], 7.71 [d, JHH ) 5.1

2

3

13 1

Hz, 2H, H3A], 7.73 [ d, JHH ) 8.5 Hz, H73A]. C{ H} NMR

150.91 MHz, 295K, CD Cl ) 18.5 [s, C78], 19.8 [s, C57], 19.8 [s,

C57], 20.4 [s, C59], 20.5 [s, C59], 20.9 [s, C58], 21.0 [s, C58],

1.3 [s, C77], 21.6 [s, C67], 21.9 [s, C68], 123.4 [s], 123.6 [s],

123.6 [s], 123.7 [s], 123.8 [s], 127.9 [s], 128.0 [s], 128.4 [s], 129.1

s], 129.2 [s], 129.4 [s], 129.9 [s], 130.5 [s], 130.7 [s], 131.4 [s],

31.4 [s], 133.7 [s], 134.2 [s], 134.2 [s], 135.7 [s], 135.8 [s], 136.8

s], 136.8 [s], 136.9 [s], 140.7 [s], 141.6 [s], 144.2 [s], 147.8 [s],

47.9 [s], 148.3 [s], 153.1 [s], 154.8 [s], 162.8 [s], 162.9 [s], 166.5

s], 166.6 [s], 166.6 [s], 166.7 [s], 170.8 [s], 171.7 [s], 173.4 [s],

81.1 [br,s, 3C], 182.4 [s, 1C], 182.7 [s, 1C], 183.4 [s, 1C], 184.3

s, 1C], 184.6 [s, 1C], 188.9 [s, 1C], 189.0 [s, 1C]. MS (LSIMS)

(

2

H55A], 7.03 [s, 1H, H63A], 7.11 [s, 1H, H63A], 7.17 [d, JHH

)

2

1

.3 Hz, 1H, H4A], 7.18 [m, 1H, H74A], 7.57 [d, JHH) 8.7 Hz,

H, H76A], 7.71 [d, JHH ) 2.3 Hz, 1H, H3A], 7.84 [d, JHH ) 7.8

2

1

3

1

Hz, 1H, H73A]. C{ H} NMR (150.91 MHz, 295K, CD

8.5 [s, C57], 19.7 [s, C59], 20.5 [s, C67], 21.0 [s, C68], 21.3 [s,

C58], 120.6 [s, C53], 123.8 [s, C4], 123.9 [s, C3], 128.1 [s, C55],

29.3 [s, C75], 129.5 [s, C63], 130.0 [s, C76], 130.8 [s, C74], 131.1

s, C73], 132.4 [s, C65], 134.3 [s, C52], 135.8 [s, C56], 135.9 [s,

C62], 136.9 [s, C51], 137.0 [s, C61], 140.8 [s, C66], 147.0 [s, C54],

48.0 [s, C64], 155.2 [s, C69], 162.9 [s, C71], 166.6 [s, C72], 181.0

2 2

Cl ) δ

[

1

[

1

[

1

[

1

[

1

+

[s, C1]. MS (LSIMS) m/z 1419.7 (M ) (calcd for M ) 1420

100%)). Anal. Calc. for C37 C, 31.31, H 1.85, N 1.97.

+

(

H

26

N

2

O

10Os

4

m/z 1433.9 (M ) (calcd for M ) 1435 (100%), 1434 (88.6%)).

Anal. Calc. for C38 C, 31.84, H 1.97, N 1.95. Found

C 32.01, H 2.22, N 2.09.

Found C 31.32, H 1.99, N 1.72.

29 2 4

H N O10Os

Solution studies: (10) (45 mg, 0.033 mmol) was dissolved in

benzene (20 mL) and heated to 200 °C in a Carius tube degassed

three times by freeze-pump-thaw techniques. After 48 h, the

reaction mixture was subjected to column chromatography and the

isolated materials corresponded to (11) (23 mg, 0.016 mmol) and

a trace of (4). Infrared spectroscopic evidence for the generation

Acknowledgment. Funding was provided by the Natural

Sciences and Engineering Council of Canada (NSERC)

through the Discovery Grants Program to J.A.C.C. and

R.K.P. J.A.C.C. acknowledges generous support from the

Canada Research Chairs Program, the Canadian Foundation

for Innovation, and the Nova Scotia Research and Innovation

Trust Fund, and ARMRC for NMR acquisition.

of a small amount of [Os (µ-H) (CO)12] was also observed as

4 4

-1

indicated by diagnostic bands at 2067 and 2019 cm . Some

insoluble black material was noted at the top of the column.

Preparation of {Os

η -C)(ηC)CHC(CH

4

(µ-H)(CO)10[η-C)N(Mes)C

2

H

4

2

NC

(µ-H)

6

H

4

2

(CH

(CO)11

3

)

2

-

2

(

3

)C

2

H

2

(η -C)]} (12). [Os

Note Added in Proof. A recent paper reported in this journal

described a DFT study on the double C-H bond activation of NHC

N-Methyl substituents with Ru3 and Os3 carbonyl clusters (Cabeza,

Javier A.; Perez-Carreno, E. Organometallics 2008, 27, 4697–4702.

(n · IMes)] (110 mg, 0.200 mmol) was dissolved in toluene (15 mL)

and heated to 200 °C in a Carius tube degassed three times by

freeze-pump-thaw techniques. After 72 h, the solution was cooled

and the solvent removed in Vacuo. The remaining solid was

2 2

dissolved in a minimum of CH Cl and applied to a silica gel

column. IR spectroscopic studies were performed on the reaction

mixtures before and after column chromatography, and these studies

Supporting Information Available: Crystallographic informa-

tion ﬁles (CIF) of the complexes and selected spectra data are

available. This material is also available free of charge via the

Internet at http://pubs.acs.org.

indicated that no rearrangements occurred on the column. {Os

C)(CO)14[η-C-η-NC (Mes)]} (8) was eluted with CH Cl /hexanes

1:4) as a very pale yellow band (6 mg, 6%). This was followed

5 5

(µ -

2

H

2

(

OM800577X

Article Doi

NHC complexes of osmium clusters: A structural and reactivity study

DOI: 10.1021/om800577x

Source and publish data:

Authors:

Article abstract of DOI:10.1021/om800577x

Full text of DOI:10.1021/om800577x

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